my friend asked me, "What is a tri?" and then asked, "is that like the Special Olympics?"
a Tri (short for Triathlon) is a 3 part race/event that has 3 portions - Swim, Bike and Run - hence the name Tri
Tri's vary in the length of the activities you have to do which determine the Name of the event.
I'll go over the common ones from longest to shortest.
1 - The most common one people hear of is the IRONMAN aka 140.6
Swim- 3.86 km (2.4 mi)
Bike - 180 km (112 mi)
Run - 42.2 km (26.2 mi) marathon
the name "140.6" comes from adding up all the miles together.
Actually Ironman is more like a brand name. There are races that equal in length but are not called "Ironman"
2 - Half Ironman aka 70.3 or Long Distance Triathlon
Swim - 1.93 km (1.2 mi)
Bike - 90 km (56 mi)
Run - 21.09 km (13.1 mi)
The name 70.3 comes from adding up all the miles together, which is Half the distance of a 140.6
Swim - 1.5 km (0.93 mi)
Bike - 40 km (24.8 mi)
Run - 10 km (6.2 mi)
4 - Sprint
Swim - 750 m (0.47 mi)
Bike - 20 km (12.4 mi)
Run - 5 km (3.1 mi)
For pool-based races a 400 or 500m swim is common. Distance can very, sprints can be considered anything under an Olympic distance