Most skills do around 40% more damage on a critical hit unless specified differently in the skill description (eg. + more damage on critical hit) or in an upgrade (eg. 250% damage on critical hit). The bonus damage % is what your gear's damage stats are multiplied with to give the actual damage digit on display. For example equipping an armor with 3.0 damage while you have 400% bonus damage would result in extra 12 damage added.