Warlock Tank (abbreviated to WT) or sometimes, "Drain Tank," is a term that refers to a warlock that frequently uses several DoT spells in unison with Dark Pact, either Siphon Life or Drain Life and Howl of Terror on several mobs at once. The term comes from the warlock's ability to sustain attacks from several enemies like a normal Tank would in a raid.
Warlock Tanks are often used in raid situations and PvP but really show their abilities in normal PvE. Warlock Tanks, because of their abilities, are easily able to kill large amounts of mobs in small amounts of time using little to no health or mana. These characters most often focus their Talent points in the Affliction Tree so to achieve talents like Dark Pact and Nightfall as well as increase the potency of the DoTs they already have with talents like Improved Corruption, Soul Siphon, Improved Howl of Terror, and Contagion.
WTs are also known for high spell resistance. They will usually wear gear that has extremely high spell resistance and carry potions that reinforce that resistance so they take minimal spell damage.
The most common WT strategy is to run through large amounts of slightly spaced mobs and continuously cast DoTs on them. When the mobs then gather, the Warlock uses Howl of Terror to send the enemies running and then uses Drain life to restore himself and drain the mobs. If Nightfall procs the warlock will quick cast Shadow Bolt as well. By the time Howl of Terror wears off, most of the mobs are dead and the warlock recharges by using Life Tap and Dark Pact on their Minion, most often times an Imp, to recharge. This method is favored by warlocks in that a single warlock is able to fight up to 12 mobs at once taking little or no damage while using little or no mana.