Hi fellow warlocks! It looks like alot of people ask the same questions over and over again so i'm going to do a small warlock faq.
Lets start with leveling.
Q: Whats the best leveling spec?
A: Affliction is amazing to level with because of the short downtime. It has alot of self-healing and you can lifetap/dark pact if you run low on mana.
I also recomend you to respec demonology when you are somewere betwin 50 and 55 because you can get the felguard.
Felguard is like an uppgraded verision of the voidwalker, he has more threat per secound, higher damage and a cleaving attack that help him hold aggro on more that 1 mobb.
Demonology is also superb for soloing group quests.
This is a good level 55 build
Q: Ok i'm now level 80 what spec should i use for PvE?
A: PVE Warlocks: A Guide to 3.1 Spex and Rotations
This will answear all your questions about pve specs.
Q: Ok i got my PvE spec, but what about PvP?
A: I'm not going to talk so much about battlegrounds here.
Destruction is the most common PvP spec at this moment but that doesn't mean you have to spec that.
Here is a common Destruction build
Destruction has alot of burst thanks to chaos bolt and conflagrate, but one of these spells can't be used if imolate isn't on the target.
The pets doesn't really matter, choose the one you prefer and benefits your team the most. If your team lack silence go for felhunter, if your team lack cc, go for succubus and if your team lack surival or spells to help you survival go for voidwalker.
Here is a good affliction build
Affliction has slow and steady damage and also self-healing throught siphon life.
The pet you want to use is felhunter because it gives you a silence and also the ability to remove magic effects.
Here is a good demonology build
I can't give much advice here but alot of your damage will come from immolation aura and your felguard.
This spec is very nice in duels against rogues because of the increased armor and reduced stun duration.
I don't suggest this spec but its fun in battlegrounds.
This specs are not nesserary the best spec for your playstyle, you could always change them abit.
Improved Healthstone vs Improved Health Funnel is one example of this. They are both nice talents and you will probably pick one of them, pick the one you like the most.
Q: Crit, haste or spirit for PvP?
A: Destruction and demonology want crit, affliction want haste or crit depending on play style.
Warlocks have pritty low crit chance even if they gem/enchant for crit, but affliction will get 9% on corruption and unstable affliction thanks to Malediction, and thats pritty much all you need.
Q:When do i use the different pets?
A:
Imp = Destruction spec in PvE (maybe for destruction after 3.2... we'll see)
Voidwalker = Some PvP situations when you need more survival, and also when you need a mini-tank for quests/mobbs.
Succubus = Some PvP situations and also when you are affliction in PvE (when doomguard is on cooldown)
Felhunter = Rarely used in PvE but very nice against casters in PvP.
Felguard = Very few situations when you don't use him, he got the highest dps/tps, a stun and cleave. Use him if you have him.
I will uppdate this post when i get more questions so feel free to ask. There are no stupid questions, only stupid answers ^^.