Summary
During the events of Ocarina of Time, the timeline splits as result of the outcome of the events. There are three known outcomes.
Child Era
The Hero succeeds but is sent back to the past by Zelda, negating the events that occurred while Link was an adult. This leads directly to the apprehension and imprisonment of Ganondorf at the Arbiter's Grounds.
Downfall Era
The Hero is defeated, leading to the Downfall branch of the timeline.
Adult Era
The Hero succeeds but does not return to the past. He is never seen again.