Life Tap is a very useful skill. It provides an immense 50% life leech, which can make most of your minions nigh unkillable while being a great aid to melee characters in your party. A well-geared Summonmancer probably won't be using the skill very often on his own, as his skeletons will be able to survive, making either Amplify Damage or Decrepify better options.
This skill allows the Necromancer to literally suck the life from his victims. The Necromancer is able to reach into the wellspring of mortality and siphon off its essence, consuming it to replace his own. Attacking a cursed soul gives you health.
↑Diablo II Manual, page 72. Blizzard Entertainment, 2000.