Time travel is the practice of displacing something in time. That is, of moving to a different time at a rate other than that of normal progression through time. There are various methods of time traveling.
The Time Traveler's Time Machine could travel freely back and forth through time, by pulling a lever backward or pushing it forward. While seated in his machine, he could see the world changing around him rapidly, as though watching a video tape in fast forward or rewind. (The Time Machine franchise: "The Time Machine", "The Time Ships")
Sometimes, travelers to pass through certain Wormholes may be able to travel through time, depending on the circumstances. For example, travelers through the Stargates (which use wormholes) may sometimes travel back through time if the wormhole's path intersects with that of a Star or other large gravitational object. (Stargate franchise: "Stargate SG-1", "Stargate Universe")
In the Xeelee Sequence, humanity creates wormholes that only link in space. Then, a ship drags one wormhole at near light speeds, so that relativity sends the wormhole into the future. This is a real observed effect. The wormhole then links the future to the present. This meant that the only way you could time travel was to the future, then back to the present.
At least in the Star Trek universe, one-way time travel through Black Holes (which, like wormholes, are a type of singularity) leads to the creation of an alternate timeline that remains separate, while two-way time travel (in which a connection to the original era is left open) can lead to the creation of a new timeline that soon merges with the original timeline and replaces much of the information there, leading to an erasure of the original timeline. (Star Trek franchise novel: "DTI: Watching the Clock")
Slingshoting around a star at superluminal speeds can also lead to time travel. The starship Enterprise was able to perform this maneuver several times. With specific calculations, they could arrive at specific moments of the past or future. (Star Trek franchise: "Star Trek")
Some extremely advanced civilizations have technology that allow them to simply materialize in a desired time, either with or without a vehicle.
One such civilization is the Time Lords. Their TARDISes can freely move through spacetime and have the ability to materialize in any time in any era. (Doctor Who franchise: "Doctor Who") Copying their enemies technology, the Daleks learnt how to develop time machines that functioned like a TARDIS, and they also developed time corridors, which was a wormhole that traversed the Time Vortex.