One of the earliest forms of time travel to be discovered involves the use of black holes. Due to their spacetime warping properties, black holes effectively send their mass backwards in time to a chronological position proportionate to the mass of the black hole. The resulting white holes appear as pulsar-like proto stars.
This is a particular form of wormhole based time travel. However, black holes are considered one-directional and therefore, less useful than bi-directional wormholes, as they effectively force a time traveller onto the changed branch of the universe, with no obvious way to return to the original branch. Stable natural bi-directional wormholes are less obvious as they functional appear like windows between two points in spacetime. They allow for access of both the original and the changed branches of the universe. The original branch exists simultaneously with the changed branch. Traversing the wormhole functions as a gateway between the two branches, which are internally consistent and do not violate any paradoxes, as both worlds continue to exist independently in the multiverse.
Sliding, also known as fifth dimensional travel, or interconditional travel, involves moving between parallel, disconnected universes and is a far more advanced feat than time travel proper.
The simplest time machine is achieved with a wormhole that has one end time dilated through something like relativistic speed travel. After sufficient effort, you eventually get one end at a position in spacetime that is very different from the other. Note however that this method is only useful for going back to a point in time after the creation of the pair of ends. As such it does not explain possible time travellers in the current human era, unless advanced alien artifacts are included in the realm of possibilities, or unless some natural wormholes exist in the universe.
Other more advanced methods may exist, but would require more sophisticated physics to understand. It may for instance, be possible to somehow move an object backwards in time using some kind of reverse time dilation. In which case you could move one end of a wormhole backwards in time, and use the result as a convenient bridge.
It is possible that a theoretical method of travelling faster than light, such as Alcubierre warp drive will actually due to relativity cause such time dilation as to reverse the flow of time locally, effectively functioning as a kind of timeship that instead of actually travelling faster than light, it will instead travel backwards in time. This could then be used to branch a second timeline by returning to subliminal speeds and proceeding normally. A timeship would not effect the causality of the original timeline, but only branch the timeline. Returning to the original timeline would likely require an Einstein-Rosen Bridge or wormhole, with one end on the ship, and the other end in the original timeline.