critical chance = the chance you have to score a critical hit.
critical damage = the extra damage your hit does when you score a critical hit.
e.g. normally you hit for 10k damage if your stat for critical damage shows 185% you'll do 18.5k damage (10000*1.85) if you score a critical hit.
let's say you have 50% critical chance on stat which means 1 out of every 2 hits will be a critical hit (in theory).
hope this helps