a flash drive is removable/portable and usually connects to a computer through a usb port. its good for transfering documents from one comp to another and stuff like that. memory in a comp is permanant so to speak. this doesnt get taken in and out. the flash drive is like a rewritable cd that u dont have to worry about scratching.
the main difference is,.. when you shut down the power. everything stored in RAM memory is lost, everything stored inflash memory however will stay stored. (untill overwrite ofcourse)