A Critical Hit increases yours damage by 100%.
Critical hit rating means the chance that yours attack will become a critical hit.
SO the higher the critical hit rating the better chance u get a critical hit.
A Critical block reduce your enemy's critical hit back to normal hit.
it may be activited only after your enemy casted a critical hit.
So the higher the critical block rating the better chance u get to block your enemy's critical hit and reduce it to normal hit.
Critical rating are effected by the following :
-The Star Spell
You can also give and receive critical heals. So basically the heal get doubled. and critical heal can not be blocked.
How to convert the rating to percentage:
Critical hit: 100 critical hit rating means 20% chance to get a critical hit
Critical block: some say 100 critical block rating means 20% chance to get a critical block