Hello all,
This guide is focused on how to enhance your Class' abilities, and how to understand them; Wich professions fit with class and specialization, what armor type is best to wear, which Stats are best for your specialiazation, and more.

1. - Armor Types.
There are 4 types of armor ingame, coming with each class and specialization. We have; Plate > Mail > Leather , and last; Cloth.
1.1 - Cloth.
Cloth users come down to three classes who can use them at best; Priest, Warlock and Mage.
Cloth focuses on Spellpower, being the source of damage/healing for the class' abilities. Best prime stat being Intellect, wich is found on all Cloth gear.
Specialization of wearing cloth is a 5% bonus to Intellect, learned on level 50 from your class trainer;

A profession for Cloth users is Tailoring. With this profession you are able to make Cloth gear from Linen to Embersilk cloth. Tailoring trainers are found in every Home City, and if you are unable to find the trainer, you can always ask a Guard for directions.

1.2 - Leather.
Leather users are primarily spread between Agility users(Melee) and Intellect users(Healing,Ranged Damage Dealer),but are not limited to using Leather only,although it gives a bonus to their abilities. Under Agility users we find Rogues and Feral Druids. Agility enhances the Rogue's/Druid's abilities by giving them Crit and Attack Power(Screenshot Below), wearing only leather provides a 5% Extra agility. Feral druids can be a Tank aswell as Melee dps. As Tank the specialization brings extra Stamina; Increasing health while in Bear form.
Intellect users,however, are bound to the Druid class only brought to you by either Restoration(Healing) or Balance(Ranged Damage Dealer).Wearing leather only provides a 5% bonus to Intellect for both of these specializations.
Rogue -

Druid -

A profession for Leather users is Leatherworking, different from Tailoring, Leatherworking uses Hides provided by a gatherer profession; Skinning. LW makes leather gear out of these Hides and leather scraps,allowing use for Druids and Rogues. Leatherworking and Skinning are both learned in any Home City.

1.3 - Mail.
Mail users are also seperated by both Agility(Though as ranged physical dps and Melee dps) and Intellect(Healing,Ranged Damage Dealer) in the Hunter and Shaman class. Hunters as whole use Agility, while in the shaman class this is only the melee Enhancement-Specialization.
Intellect is brought only to shamans as Elemental(Damage) or Restoration(Healing)
Hunter -

Shaman -

Mail users are also bound to Leatherworking, as it makes Mail gear aswell as Leather. Blacksmithing makes Mail gear aswell as Plate gear.

1.4 - Plate.
Plate users are seperated by the Paladin, Death Knight and Warrior class. Plate classes primarily are melee dps or Tank, with the exception of the Holy paladin specialazation(Healing).
Death Knights having Unholy and Frost as dps specialization, Warriors having Arms and Fury and Paladins having Retribution. All of these specializations give 5% bonus to Strengt when wearing full plate.
Each class also comes with a Tanking spec; Death knights having Blood, and Warriors/Paladins having Protection. Wearing full plate is advised for these Tank classes as it increases armor, and Armor reduces physical damage taken. Wearing full plate as Tank gives a 5% bonus to Stamina.
Holy Paladins are the last spec not mentioned, Holy is a healing specialization, based on Intellect. Wearing full plate will give a 5% bonus to it.
Death Knights -

Warriors -

Paladins -

Plate gear is made with Blacksmithing. BS uses mineral bars from Mining(Gatherer profession) and its trustwhorty Blacksmithing Hammer to bend the bars into powerful armor.

2. - Class Roles.
I've mentioned them before, there are 3 roles in-game; DPS(also known as Damage Dealer, stands for *DamagePerSecond*), Tank and Healer.

2.1 - DPS.
Every class can deal damage and has the appropiate specialization to it. There are, however 2 different kinds of damage dealers wich can also be split into 2 subclasses.
There are Melee DPS, and Ranged DPS; Melee dps fight close to the boss, with their weapons. Two kinds of melee dps are around at the time of speaking, those based on Agility(Enh.Shaman,Rogue,Feral Druid) and Strenght based dps(Death Knights, Warriors,Paladins).
Hunters are the only physical ranged dps class to date, meaning they are based on Agility rather then spellpower(Int).All other ranged dps,however, are spellcasters(Ele.Shamans,Warlocks,Priests,Mages,BalanceDruids).

2.2 - Tanks
Tanks take the damage in the game, distracting the enemy so DPS have an easy time bringing him/her/it down. Most tanks are Plate wearers, yet Feral druids transform to Bear form to take the damage. Tanks go for avoidance(dodge/parry), stamina and Mastery. Stamina increases health, and Mastery reduces incoming Physical damage (Palladin/Warrior with Block, Death knights with Blood shield, Ferals with Savage Defender).

2.3 - Healers
Healers heal. Simple as that! Healing incoming damage while maintaining their resources(mana) is the goal of healers, as a dead player, isn't usefull.Intellect is a stat that is on all healer gear, but another stat is probably worth a mention; Spirit. Spirit gives mana per second back in combat, and is extremely usefull as an OOM(OutOfMana) healer will most of the time lead to a wipe.
Healing classes/specs are Restoration Druids/Shamans, Holy Paladins and Holy/Discipline priest.

Chosing what role fits best to you may be hard, but you'll definatly find out after playing the possibilities.

3. - Stats!
Stats are subclassed in Prime, and Secondary stats. They are split because of reforging. Reforging allows to take 40% off of a secondary stat on your gear, and put it in another stat.
3.1 - Prime Stats
1- Stamina. Mentioned before, stamina increases health per point., and is on most, if not all gear.
2- Intellect. Also formentioned, Intellect increases Spellpower, aswell as your mana pool.
3- Agility. Agility increases Attack power and crit for Rogue/druid/hunter/shaman classes/specs, but does mostly nothing for classes wich can not use it to its benefits.
4- Strenght. Lastly, strenght grants attack power for the Plate melee classes, but is also on Tank gear, for the use of threat.

3.2 - Secondary Stats
1-Haste. Haste decreases time between attacks for melee, and decreases cast time for casters. It also benefits Hunters,Deathknights, Rogues and feral druids for the regeneration of their resource system, more on that below.
2-Crit. Crit increases the chance of your heals/damaging abilities giving a critical strike per point. A critical strike is seen as 200% of normal damage, before modified by talents.
3-Spirit. Spirit is usefull for healers , granting mana back per second in combat. Spirit is also used as hit by Shadow Priests and Balance druids, modified by talents.
4-Dodge/Parry(Miss). Avoidance is the name of these stats combined. They increase the chance you dodge , or parry the attack, completely mitigating incoming damage. You also have a chance of being missed by an attack, but you can not gear/reforge to this stat.
5-Expertise. Expertise is used by melee classes to be able to attack the boss from behind. A rating of 26 Expertise makes it able to have your attacks not dodged from behind. You need 56 Expertise to be able to attack the boss without being parried from the front, but this cap is hard to get to so most players go for the simple 26 cap and attack the boss from behind.
6-Hit rating. Hit is a stat all but healers need. Hit makes it possible for your abilities to actually -hit- the enemy. For most classes the hitcap is different as of hit-modifying talents.
7-Mastery. Mastery is different for every class. What mastery rating does for your class can be seen in your Talent tree tab, under *Summaries*. Mastery can only be learned at lvl 80.

Finding out wich stats are best for your specialization may seem hard, but testing them out in combat, or a training dummy can provide enough info to make your character stronger.
Also- If you can't find out yourself, there is valuable info on the internet, on sites as MMOChampion,Elitistjerks, and probably more.

4.Your Resource System.
All, if not most of your abilities cost something. This *Something* is your resource system.
4.1 - Mana.
Mana is used by all casters, and is modified by intellect. Caster abilities cost a percentage of your base mana. Base mana means your mana pool without any gear equipped, or modified by talents.

4.2 - Energy
Energy is used by feral druids(DPS) and rogues. They have talents that increase your energy regeneration. Not only energy is used by these specs, but also Combo points. Some abilities do increased damage based on the amount of points on the target. Most attacks give one if not two combo points. Using abilities when you are at highest amount of Combo points(5) grants you the most damage done.

4.3 - Rage
Rage is used by Feral druid(tanks) and warriors. Damage done to you aswell as your own abilities grant rage, and rage itself is used for other abilities. You start the fight on 0 Rage, unlike with energy, where you start on 100 waiting for it to recharge.

4.4 - Unique Resource Systems.
Some classes come with their own resource system, I'll mention them right now.
1.Paladins - Most paladin abilities grant a point of Holy Power. Unleashing this holy power on the highest amount of stacks (3) is best for outgoing damage/healing. Holy power is used for certain paladin abilities like Templars Verdict and World of Glory.

2.Balance Druids- Balance druids have a system called Lunar Eclipse. Its much like holy power only with a different skin(Correct me if i'm wrong though, this might be the only spec ingame I never played).

3. Hunters- Hunters use Focus for their abilities. Its much like Energy, only it doesnt use Combo Points.

4.Warlocks- Warlocks use Soul Shards to enhance their abilities. They only have 3. Using a Soul shard makes you able to change certain abilities of yours, including instant pet summoning. Recharging Soul shards can only be done through Soul Drain or Soul Harvest.

5.Death knights- Death knights probably have the most unique resource system of all. Abilities cost either Blood, Frost or Unholy runes. and you'll have a set of 2 for each of them. They recharge based on haste, and have a chance of completely recharging after using an ability based runic power. Runic power is much like rage, its gain by using the above-mentioned Rune-Abilities and is used for abilities like Death Coil, Rune Strike and Frost Strike.

5.Professions! YIKES!
Professions are used to either have fun, or enhance your player abilities. There are 3 kinds of Professions; Gatherer Professions, Secondary Professions and a group wich are individually different from eachother. You can only have 2 Professions at a time. Secondary professions,however, are able to have as a plus for every char.
5.1 Gatherer Professions.
Gatherer professions are Mining, Herbalism and Skinning. They *gather* stuff by mining veins, picking up herbs, or skinning the hides of dead animals. These are used in professions like Blacksmithing,Engineering,Jewelcrafting, Alchemy, Inscription and Leatherworking, and can be used in favor of these professions or simple Auction House selling. Mining also gives a stamina bonus, Herbalism a haste buff/every 2minutes and skinning gives a bonus to Crit rating.
5.2 Secondary Professions
There are 4 secondary professions ingame;
1-First Aid. First aid uses bandages made from clothes to heal your wounds, and makes anti potions aswell.
2-Cooking. Cooking makes food from every meat,fish, beetle etcetera you find ingame. Some cooked food enhances your player stats, wich is what makes cooking worth taking.
3-Fishing. Definatly the profession that takes most time to cap. Fishing, well. Fish!. You fish up fish used in Cooking recipies and sometimes you can fish up some fun vanity items...
4- Archaeology. Archeaelogy makes you run around the world in search of artifacts. When you first learn it, you get 4 Digsites in every continent. In those digsites you can use an ability called *Survey* wich points you to the direction of artifact fragments. Archaeology's only beneficial part is that it has lots of very neat and fun vanity items.
Lastly, There is the riding profession, wich allows you to mount, being able to run/fly faster. You cant train it,however, you can increase your runspeed for gold at Riding Trainers at level 20, 40, 60
and up.
5.3 - All that is left behind, but not forgotten.
These last professions can enhance your character the most, and are definatly a plus in PvE-Content.
1-Blacksmithing. BS uses mineral bars to make Plate and Mail gear. It also gives 2 extra sockets on high level, allowing to put Gems in(more on that below).
2-Leatherworking. LW Uses the hides of animals to make Leather and Mail gear, It gives an LW-Only enchant on your bracers on high level.
3-Tailoring. Tailoring uses cloths found on most humanoid mobs to make cloth gear. On higher levels they give Tailor-Only cloak enchants, wich are, quite nice.
4-Jewelcrafting. JC uses ores to prospect for Gems, and make these gems for sockets wich are on most gear. Gems can be equipped in these sockets to enhance your stats.It also makes Rings, Trinkets and Neck-jewelry.
5-Alchemy. Alchemists use herbs to make Elixirs,Flasks and Potions wich increase your stats. They also do transmutes, these transmuted elements are used in lots of professions and are worth a nice chunk of gold.
6-Inscription. Scribes use herbs to make glyphs. Glyphs are used by every class to enhance your abilities, you can see the list of glyphes available to you in your Talent tree tab.
7-Enchanting. Enchanters uses dust,shards and crystals to enchant gear, increasing your stats. These dusts, crystals and shards are obtained by disenchanting gear.
8-Engineering. Engineering has lots of cool stuf from tinker enchants,to pets, to mounts!. Engineering uses bars from mining, and is really the Tech-based profession of the game.

Thats it! I hope you enjoyed this little Guide aswell as I did writing it,