A triathlon is an athletic contest in which participants compete three events in succession. Usually these events are swimming, bicycling and running.

There are no set distances for triathlons. Many triathlons use various distances that conform to the land/water available to them.

There are, however, a few "common" distances:

Sprint: .75km (swim), 20km (bike), 5km (run)
Olympic (International): 1.5km (swim), 40km (bike), 10km (run)
Half Iron Man: 1.2m (swim), 56m (bike), 13.1m (run)
Iron Man: 2.4m (swim), 112m (bike), 26.1m (run)

A duathlon is an athletic contest in which participants compete two events in succession. Usually these events are bicycling and running. Basically a duathlon is a triathlon without the swimming. Duathlons often split up the run so as to be a run-bike-run event.