Circuit breaker kicks off when one switch is on
What you're describing is known as a "switch loop".  A cable runs between the switch box and the fixture box, which is used to interrupt the ungrounded (hot) conductor.

Basically, power comes into the box on the black wire in the "feed cable".  It travels along the white wire in the "* switch cable*" to the switch, then back from the switch on the black wire in the "switch cable" to the fixture.