If you critical on a spell, the spell does twice ( 2x ) the amount of damage it is supposed to. Critical rating is the amount of times that you will critical. I believe if you pass 350 critical rating in any school, you will always critical with that certain school spell (also i'm not sure) Critical block does the exact opposite.
The common mistake I see people making is thinking that 600 critical for example is the percent chance they have. This is not the case. Rating is converted to a percent for each stat on an item. The larger the rating is the higher the percent it will be converted to. Fusilier's Fireberry Saber has 125 critical rating and Hades' Flamemaker has 40 universal critical rating and 86 fire critical rating. They are close to the same amount so its good to use these for an example. Fireberry has all its critical in a single chunk which turns to 23 percent. Hades staff is in 2 parts which convert into 4 percent and 14 percent for a total of 18 percent. So we see here that the Fireberry offers 5 percent more critical than the hades staff even though the ratings are 125 (Fireberry) and 126 (Hades). So when selecting gear for critical watch for larger chunks of rating as they are converted to more percent. What this means is somebody can have less rating displayed as another person but actually have a better chance to critical. As far as blocking goes. If you had 100 percent chance to critical and the target has 20 percent chance to block then you will for sure have a spell that criticals however the chance it goes through is only gonna be 80 percent since they are blocking 20. Hope this clears things up. When you see a high rating you can check to see if the persons pet is boosting it. A quad critical pet can make a person look like they have amazing critical chance but at max player level each of those pet talents get converted to 1 percent maybe 2 percent tops on a couple of them. You can mouse over your ratings for critical and block and see the percents broken down for you. Just add these numbers up for a stat to see what the actual percentage is.