Attack rating is how big your chance of killing an enemy is (relative to their defence rating)
Charge applies for the first few seconds after combat is started (when you tell a unit to attck another - you dont get it if you are attacked from the side or rear or by surprise). AFAIK the charge number is a bonus added to your attack rating for this short period of time.
Charge can make all the difference especially for cavalry and units like woodsmen who have low attack but a huge charge.
A question for others..... is the negative moral modifier you get for being hit in the flank or rear more when the enemy unit is charging rather than just attcking you?