Sontag, Susan. “Notes on Camp.” From Against Interpretation: And Other Essays. New York: Dell Pub. Co, 1966.
If I may attempt to summarize a definition Sontag spends the entire essay grappling with, Camp is art, broadly defined, created with seriousness whose intended content is a failure and is thus appreciated, with nostalgia, ironic humor, and not too much cyncism, purely for its style.
It is almost impossible to intentionally produce camp, at least it is impossible to intentionally produce good camp.
Sontag's written reminded me of Bill Simmons's appreciation for "unintentional comedy."
She places homosexuals at the vanguard of camp taste, acknowledging that there is something propagandistic about that position. (That reminds me of the Simpsons episode where Homer deals with his homophobia as he interacts with the owner of a store that sells Camp-y knick-knacks.)
If anything, Camp has become more mainstream in the half-century since this essay was written. See: the Internet especially YouTube.