You have normal crits when you're in normal battle. Like when you are face to face bashing a monsters head in, you could crit for 50%(default) extra damage. <--- these are dependent on crit chance (crit rating)

The stealth crits are if you are undetected and to the side of or behind an enemy. You get an even larger crit. I think it is 100% more damage from a stealth opener. <-- these are guaranteed (if positioned correctly)

Every attack has a chance to cause critical damage.... When you attack from stealth, you are guaranteed to cause critical damage.
The spell & weapon critical stat determines your characters chance of causing critical damage when not in stealth.