Crit Immune

http://www.wowwiki.com/Defense  states you need (at 80) 540 defense rating to be "crit immune".

Since mobs have around 5% chance to crit, you need 5% chance to avoid being crit, which is obtained by 540 def

Defense rating/5=Defense

To find out how much defense an item will give you, take the defense rating it has and divide it by 5. That will show you how much defense skill it will give you.

Another way to look at it is:

5 defense rating=1 defense skill