Life Tap is an ability that transforms a portion of the warlock's own life force into fel power. It seems to be a variation of the Death Knights' Unholy Armor spell. This ancient Necromantic spell transforms a portion of the recipient's life force into something unearthly and seems to be a variation of the Necrolytes' Unholy Armor spell.
The most dedicated of the dark arts are willing to sacrifice everything in their pursuit of greater power, and this spell is one example of how far some warlocks are willing to go. When casting this spell, the caster cuts a gash in the body, using blood as a reagent.
Warlocks in a party with a healer can boost their mana if the healer agrees. Note that the self-inflicted damage will rank low on the healers priority list - after the tank and party members taking aggro.
Between fights, use Life Tap to bring your life and mana even. This way, you will regenerate both hp and mana at the same time, and will decrease your downtime.
While Life Tap will allow you to continue casting, it does trigger your GCD, reducing your overall DPS, and should be used sparingly.
Life Tap does not generate aggro.
Patch 7.0.3 (19-Jul-2016): Now available at level 34 for all specializations (core ability).
Patch 6.0.2 (14-October-2014): Now available at level 16 for those who have chosen the Affliction or Demonology specialization.
Patch 3.3.3 (2010-03-23): This spell no longer scales with spirit, and instead scales with spell power.