How does "life steal" work?

Life Steal returns health to you equal to 10% of the damage you deal to your enemies' health bar (after armour has been eliminated - Life Steal has no affect while armour is up).

Life Steal from a weapon mod stacks with Life Steal from a piece of wargear, so if you get 10% from a weapon, for example, and 10% from wargear, your Life Steal is 20% cumulatively.