A triathlon is an endurance event consisting of swimming, cycling and running over various distances.  These events are executed back-to-back and the official time for the event will include the transition between individual legs.

A "standard" or "Olympic" triathlon consists of a 1.5km swim, 40km bike ride and 10km run.  A "sprint" triathlon would be half of that.

Triathlons are run under the rules of the British Triathlon Federation, whose web site contains more information.