Impostor, faker, quack, mountebank, charlatan denote a person who makes pretensions to being someone or something that he is not or of being able to do something he cannot really do.
Impostor applies especially to one who passes himself off for someone else. However the word often serves as a general term for anyone who assumes a title, character, or profession that is not his own.
Faker applies to one who gives himself the appearance of being what, in character or in profession, he is not.
Quack is the popular and contemptuous term for an ignorant, untrained, or unscrupulous practitioner of medicine or law or seller of remedies or treatments, and usually carries a strong implication of fraud or self-delusion.
Mountebank sometimes suggests quackery, but it regularly suggests cheap and undignified efforts to win attention.
Charlatan applies to a writer, speaker, preacher, professor, or expert who covers his ignorance or lack of skill by pretentious, flashy, or magniloquent display.