Critical Rating adds to your chance for a critical. Critical always does double total damage if not Blocked (after all traps/blades/shields/resist). The rating system is kinda complex as it changes from level to level. The easiest thing to do is ignore the ratings and look at the mouseover of the rating itself. It will show the % chances each item gives you. Add those percentages and you have your total chance to critical.
If you want a more detailed explaination read these.
Critical rating is te chance that your spell will do double the usual damage. Along with critical rating is critical block rating which is the chance that you or your enemy will block the critical and the spell does normal damage. You must be at least level fifty to get gear with critical and critical block rating. Good Luck! Saffron Sandsword Legendary
Critical rating, which is unlocked (pets, Winterbane, and Pagoda gear excluded) at level 50, gives a wizard or monster the ability to do twice the damage of a spell. Diego the Duelmaster will reiterate this when it is unlocked. The counter to critical rating is critical block, which can remove the power of critical. When fighting an opponent without critical, critical block is useless. That is, critical block will not activate without critical. Different gear adds different levels of critical. Storm wizards are generally associated with having the most critical, and surprisingly (given that they are crafted to be a defensive school) life is in second. Ice is able to have the most critical block. Hope it helps!