Cam had been in two relationships with Booth; once before he worked with Bones and after she came to the Jeffersonian. After they broke up, they continued to work together and are now just friends.
Camille and Booth have had an intimate/romantic relationship in the past, but he did not want to marry her.
Cam has known Booth and his younger brother, Jared, for fifteen years, which seems to support this conclusion or at least indicate they knew each other when they were teens.
In The Truth in the Lye, they slept together at the end of the episode. It is revealed however in Aliens in a Spaceship, that they had struck up their relationship again after she came to the Jeffersonian, although Booth appeared to have some reservations and was uncomfortable telling anyone at work, especially Dr. Brennan, about his connection with Camille. In The Man in the Cell, Cam inhales a toxin while performing an autopsy, which puts her life in danger. After this case, Booth broke up with Cam when he asserted workplace relationships put everyone in danger when high-pressure situations arise.