An imposter is a person who pretends to be someone else. Someone who tries to convince you that he's your long lost cousin in order to get an invitation to stay in your awesome apartment is an imposter.
An imposter usually seeks some kind of financial gain when he or she assumes another person's identity, but there can be other motivations, like simply the thrill of doing it. Any imposter who pretends to be the sibling of a famous person, for instance, will get lots of attention. The Latin root is impostorem, "impose upon or deceive." It's more commonly spelled impostor, although both spellings are correct.