A Part File (.ipt) is a single file containing one part. This part is created by a feature or multiple features.
An Assembly File (.iam) is a file containing the parts that you place. These parts are then constrained to each other to form the desired assembly. An assembly file is useless without the associated part files/ links. Hence the problem that arises when people attempt to post their Assembly Files on here for help and no one can open them up because the associated Part Files have not been included. Hope this helps.
