What is, exactly, a triathlon?

A triathlon is a race made of three even segments, a swim, a bike, and a run.
There are three main distance races:
~Ironman (the original): Swim 3.8 km(2.4 mi); Bike 180 km (112 mi); Run 42.2 km (26.2 mi)
~Olympic: Swim 1.5 km (0.93 mi); Bike 40 km (24.8 mi); Run 10 km (6.2 mi)
~Sprint: Swim 750 m (0.5 mi); Bike 20 km (12.4 mi); Run 5 km (3.1 mi)

I'm doing the Philadelphia Insurance Olympic Distance tri on June 22.