“Triathlon is an exciting multi-discipline sport involving a continuous race over various distances in the three disciplines of swimming, cycling and running. A standard triathlon is made up of a swim, followed by a cycle ride, followed by a run. Competitors race against the clock, which starts as they enter the swim and stops as they cross the finish line after the run. For this reason, triathlon is often reported as having a ‘fourth discipline’ known as the transition. The transition is the point in the race when competitors change from swimming to cycling, and from cycling to running.”

Source: British Triathlon

Triathlon races can take place over a number of different distances, the standard distances are:

Type Swim Bike Run
Super Sprint 400m 10k 2.5km
Sprint Distance 750m 20km 5km
Standard Distance 1500m 40km 10km
Middle Distance 2.5km 80km 20km
Ironman 70.3 1.9km 90km 21km
Ironman Distance 3.8km 180km 42km

Triathlon can be split into two parts, ITU racing (predominantly Standard distance also known as Olympic distance) and Ironman racing (Ironman distance and 70.3 distance. 70.3 is half of the Ironman distance).  As well as the ITU and WTC there are also lots of individual race organisers who offer similar distance events but whose races sit outside these two groups.