Affliction

The Affliction Warlock excels at doing single-target damage and is also one of the best multi-target specialization in the current game, as well as being capable of being a top end single target spec on fights with frequent adds.
Affliction warlocks are the king of dots in WoW, with up to 5 damage over time effects possible, as well as being extremely effective at applying damage to 5+ targets at once, as well as being able to completely change their playstyle on spec to being a single target monster. They are also extremely mobile due to much of this damage being instant cast dots.

 

An extremely useful tool for overall optimization

http://www.askmrrobot.com

Wowhead Affliction guide

http://www.wowhead.com/affliction-warlock-guide

Icy Veins Affliction guide

https://www.icy-veins.com/wow/affliction-warlock-pve-dps-guide

Noxxic Affliction guide

http://www.noxxic.com/wow/pve/warlock/affliction/