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
Wowhead Affliction guide
Icy Veins Affliction guide
Noxxic Affliction guide