Olympic: 1500m swim, 40km bike, 10km run. The standard Olympic and World Triathlon Series distance is a somewhat longer race, but a good test of speed and endurance. Finish times are generally ~2-3 hours.
Middle Distance/Half Ironman (or Ironman 70.3): 1900m swim, 90km bike, 21km run. This is where life gets a bit more grueling, but a great distance as it’s long enough to make a good destination event, but short enough to train for alongside a normal life. Finish times are generally ~4.5-6.5 hours.
Full Distance/Long Distance/Ironman: 3800m swim, 180km bike, 42km run. This is a serious undertaking and one of the hardest one-day events. It’s a defining achievement for some and an addiction for others. Expect training to take over your life for 6 months prior to the race. Finish times range from 8 hours (gods) to 10-14 hours (mortals) and up to 17 hours (the cut-off for those just wanting to finish).