Critical Hit Rating increase the player's chance of getting a critical hit. Block Rating increases the player's chance of avoiding incoming critical hits. Players will gain Critical Hit and Block bonuses from equipment.
You will also notice with these Critical and Block Statistics, that the Combat Rating System displays as a numeric value and not a percentage.
100 Critical rating does not equal 100% chance of casting a critical attack.
I was confused about this not too long ago. Here's a tip. If you mouse over your critical rating, you will see all of your gear that adds critical rating and a percent value. If you add up all of these percents, that is approx your chance of critting. Hope this helped! Jacob Darkbringer Grandmaster