It both increases your own crit rate and decreases the crit rate of whoever is attacking you. I've been told that 100 CRI basically makes a 10% difference in crit rate, though I have not attempted to independently verify that.

Some characters naturally have much higher CRI than others. Some can easily end up with CRI around 2500, while others won't go far over 1000 unless you go out of your way to stack a bunch of CRI random stats on gear.