[{quoted}](name=King KSer,realm=NA,application-id=3ErqAdtq,discussion-id=Z3pIIrbA,comment-id=0001,timestamp=2016-03-01T06:33:40.465+0000)
Life steal is based off the percentage of damage you deal. The more damage you deal, the more you life steal.
Percentage of the PHYSICAL damage you deal FROM AUTO ATTACKS.
Magic damage from wit's end, devourer, or guinsoo's rage doesn't count
Physical damage from abilities like zed's shurikens doesn't count
But your auto damage counts, bonus crit damage counts, on hits count ONLY IF THEY DEAL PHYSICAL DAMAGE. That means Fervor procs count, Nasus's Q counts, abilities that apply on-hits like Ezreal Q and Yasuo Q count.
There are some exceptions to these rules, like Hydra making lifesteal apply to its active or Aatrox's on-hit damage toggle not applying lifesteal, and some vaguely defined abilities that you need to specifically check if they count as auto attacks or not.