Thanks a lot for the very informative post. I hope it will take very very long time until you complete your skill trees...maybe months so it will feel like a worthy and long goal among all othersYou will be able to earn enough skill points to unlock every ability in the game. Some skills will offer morph options so they split into two trees. You can only play one morph tree at a time but you can respec to change your morph tree. They said however far along you are in the morph tree (let's say you completed it) if you later respec to go in to the other morph tree, you will then have the same progress in that - so all of it would be unlocked in this example.
So you can end up when you are finally done progressing your character with skills for every armor type and every weapon type, all your class skills, all of the skills available through guilds and pvp. Also there are world skills you can find which are hidden (such as the trees for vampirism and lycanthropy).
Deck building is selecting which skills you have amassed in your arsenal to go on your hot bar which is limited to 5 skills plus an ultimate. These skills will then be the ones you use in combat until you swap them out when out of combat. At level 15 you unlock weapon swapping. Your second weapon will have a different set of abilities on its hotbar. So in essence, you can swap between these two sets of skills on your hot bar as you swap weapons. So you would have 10 skills plus 2 ultimates to play with during battles.
Also, there's no global cooldown or cooldown of any sort on your skills. You have to manage your resources. You could spam your skills but once you use up your stamina or magicka you are screwed. So you have to be careful about how spammy you are.
You get rewarded by how well you perform in combat. This charges your finesse (which fuels the ultimate), grants extra loot and sometimes the kill cam. At low levels it is harder to get the finesse bonus if you are grouped but grouping will grant you extra xp and easier time progressing through content. At higher levels, it will be easier to earn finesse in groups doing dungeons because those mobs are harder to kill.