A triathlon is a multi sport event. You have to complete three continuous and sequential endurance events. Triathlon, in its most popular form, involves swimming, cycling and running over various distances. Triathletes compete for fastest overall course completion time, including timed "transitions" between the individual swim, bike, and run components.
Triathlon races vary in distance. The distances are Sprint distance (750 m swim, 20 km bike, 5 km run), Intermediate (or Standard) distance, commonly referred to as "Olympic distance" (1.5 km swim, 40 km ride, 10 km run), the Long Course (1.9 km swim, 90 km ride, 21.1 km run, such as the Half Ironman), and Ultra Distance (3.8 km swim, 180 km ride, and a marathon: 42.2 km run); the most recognized branded Ultra Distance is the famous Iron Man Triathlon
Transition areas are placed between the swim and bike segments (T1), and between the bike and run segments (T2). This is where you swap your gear and change into the next components. The time spent in T1 and T2 are included in the overall time of the race.