General

Well, there's critical rate, and critical damage rate. Critical rate increases (or decreases) the chance of hitting a critical attack. A critical attack is just an enhanced attack that hits higher. Minimum and maximum crit rates set the boundaries of attack ranges.

All classes have a base 120% (125%?) Minimum critical damage and 150% Maximum critical damage

So when you hit a critical you will do extra damage within those 2 ranges. Now say that you had an extra 20% minimum critical damage, then you'd have 140% Minimum and 150% maximum, meaning more consistently high critical hits.

It's basically like how Mastery affects your minimum damage range.