Triathlon and time trial bikes are designed to optimise aerodynamics. In most trathlon races, riding in a peloton (a pack of riders with one at the front bearing the brunt of the wind resistance) is not allowed so the bikes used more closely resemble individual time trial races.
The Triathlon and time-trail bikes' most distinguishing feature is special handle bars called 'aero-bars' or 'tri-bars', set lower than the back end, which hold your forearms in a position that makes your front-on body profile smaller to cause the minimum amount of drag possible.
To spare muscle groups used for your last triathlon event, the run, the frame geometry includes a steep seat-tube angle, which also improves aerodynamics.
