Life Steal

Life Steal is a modifier that converts a percentage of the damage dealt into Life regained for the attacker. Life Regained = % of Life Steal x Total Damage Dealt.

Life Steal can be extremely effective when combined with high damaging attacks. The greater the damage a character deals, the more life the character will regain. Life Steal can be a huge factor in deciding the outcome of a battle when fighting Elite Monsters, Bosses and especially while fighting in the PvP Arena.

A Character's Life Steal can be increased by the Gear that he or she equips and by the Skills that he or she decides to use.

Also See:
Healing
Life Regeneration
Life Per Hit 
Increase Armor
Reduce Damage Taken
Increase Attack Speed

The Following Is A List Of Each Class' Skills That Increase Life Steal:

Barbarian Life Steal Skills

Ignore Pain (Ignorance is Bliss)
Rend (Blood Lust)
Ancient Spear (Siphon)
Bloodthirst (Passive Skill) 

Demon Hunter Life Steal Skills

Entangling Shot (Bounty Hunter)
Elemental Arrow (Nether Tentacles)
Shadow Power (Blood Moon)
Marked for Death (Death Toll)
Cluster Arrow (Maelstrom)

Witch Doctor Life Steal Skills

Summon Zombie Dogs (Leeching Beasts)
Haunt (Consuming Spirit)
Firebats (Vampire Bats)
Spirit Barrage (Phlebotomize)

Wizard Life Steal Skills

Magic Weapon (Blood Magic)

Check Out This Informative Video On Life Per Hit Vs. Life Steal!

Note: The more damage you do will directly correlate to a higher Life Gain per attack. Increased Attack Speed will increase the total number of hits you do per minute, which will ultimately lead to a greater total life gain. Since Life Steal is a % of damage done you will want to have at least 2.5% Life Steal and a high DPS weapon. Also make sure to get gear with your character's offensive attribute (Eg. Strength, Dexterity, Intelligence)