Most/all SLRs and compacts are threaded 1/4" UNC. Some larger (medium format and above) cameras are 3/8" UNC.
Some tripods have only a 1/4" screw, but others come with a 3/8" adapter.
EDIT/ There's an echo in 'ere.

P.S. UNC and Whitworth are just about identical - easier to get UNC nuts/bolts these days.
