To dress and behave in a certain way often happens in order to be something: a man, a woman, heterosexual, butch etc. This is possible because these actions refer to a system of officially approved signs, on the base of which people generate their sexual, gendered, ethnicised etc. identity. But “[t]here is no Gender identity behind the expressions of gender; ... identity is performatively constituted by the very "expressions" that are said to be its results.' (Butler 1990: 25). In other words, gender, sexuality, Ethnicity etc. are performances; it's what people do at particular times, rather than a universal who they are.
Seen in this way, identities, gendered and otherwise, do not express some authentic inner "core" self but are the dramatic effect (rather than the cause) of performances.

_{Butler, J. (1990): Gender Trouble, London, New York}