(despite these other defintions, booty does not just mean an ass, thats what the word ass is for)
Booty means a perky, rounded ass, usually applied to women and used to describe said ass' high desireability. The ass can be bigger than average but not too big. See also bootay
Note that it doesn't apply to an ass that is just fat or big, for that see fatass
or just look at yo momma.