When the dark arts call for flesh, you can only trust your own.
Life Tap is a core warlock ability learned at level 16. It converts health into mana. It is somewhat of a signature ability of the warlock.
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 3.3.3 (2010-03-23): This spell no longer scales with spirit, and instead scales with spell power.