There are various Timelines on Fringe. While the history of the timelines is quite complex and hasn't been fully explored on the show, the following is understood.
First Timeline (Unfolds Once)Edit
The first timeline occurred naturally and is never seen on screen. Observers were created in 2167. By the 2600's, a science team is sent back to observe their beginnings, leading directly to the Original Timeline.
Original Timeline (Loops Several Times)Edit
See Original Timeline for complete list of events.
The Original Timeline is called this because it is the first timeline to be shown on the television series. The timeline strays from the First Timeline when September accidentally distracts Walternate from curing Peter. This timeline ends around 2026. The First People send the Machine back in time creating a time loop. It is never specified how many times this timeline loops, but it has looped at least once and may have looped an infinte number of times. Eventually, Peter closes the loops by making The Bridge instead of destroying the Alternate Universe.
Alternate Timeline (Unfolds Once)Edit
See Alternate Timeline for complete list of events.
The Alternate Timeline replaces the Original Timeline after Peter gets in The Machine. Chronologically, though, it occurs after the Original Timeline. The Alternate Timeline holds true in the Final Timeline until 2015 at the moment of the invasion.
Final Timeline (Unfolds Once)Edit
See Final Timeline for complete list of events.
After Walter's Plan succeeds, the timeline loops back to 2015, erasing all events that happened in the Alternate Timeline during and after The Invasion.