A proc is a specialized spell that can be in a buff or as an effect on an item. A proc is usually given a percentage chance to go active and can be triggered by successful attacks, combat arts or hostile spells. They also can be triggered when the owner of the proc is themself struck by a weapon or hostile spell or casts a beneficial spell on someone else. If a percentage chance is not listed than it is assumed to trigger all the time. Only one proc of any time can trigger. The formula for determining which proc triggers is not known.
The percentage chance listed is modified by the delay of the weapon or casting time of the spell or combat art. This means that a 10% proc on a weapon will trigger the same number of times per minute, on average, regardless of the speed of the weapon. The following formulae can be used for calculating the actual chance to proc:
Autoattack Proc Chance = (Delay / 3) * Proc%
Example: A 2.4 delay weapon w/ a 10% proc actually has a (2.4/3)*10% or 8% chance to proc on an autoattack
Combat Art Proc Chance = (Casting Time / 3) * Proc%
Example: Using Agony, a 0.5s casting time combat art with the same 10% proc weapon as above only has a (0.5/3)*10% or about a 1.7% chance to proc.
Spell Proc Chance = (Casting Time / 3) * Proc%
Example: Using Baleful Wrath, a 2 second casting time spell while wearing the Mystical Orb of the Invoker that has a 10% proc on hostile spells only has a (2.0/3)*10% or 6.7% chance to proc when the spell is cast.