Time Travel stories generally centre around a technology or device that allows travel through time. This particular sub genre can occasionally cross over with Alternate History SF although in the majority of those cases, the main theme of the story is the restoration of the original time line. The 'Back to the Future' trilogy of movies is a good example of this.

Some Time Travel SF, such as 'Doctor Who' and 'Quantum Leap', centres around an individual time traveller who must make alterations in the part (or the future) to ensure that people can carry on their lives as normal, often without even realising that their way of life had been put in jeopardy.

In many Time Travel stories, there are fixed points in time that cannot be altered for any reason, even if that means the death of a loved one of the destruction of a species. If these fixed points in time are somehow altered, often that ocurrs with disastrous consequences and things are worse because of the alteration.

In many ways Time Travel stories are quite difficult to write. For example you cannot just dive into history, storm the Third Reich and kill Hitler. First and foremost you must think what the consequences of that action might be. Had Hitler been murdered in 1938, what would the world look like today? What happened between then and now to make the world as you have created it? Would Germany exist as a whole, or would the Berlin Wall still stand?

An addition to this little definition, thanks to KiplingKat...

Some Time Travel SF features the use of Time Loops. Such stories generally feature a character who must live the same day over and over, until he/she learns a particular lesson or carries out a task in a certain way.

Other examples of Time Travel SF are:

The ITV show 'Primeval', and the accompanying novels - All of which I highly recommend.

The 'Tikka to Ride' episode of the BBC show, 'Red Dwarf'.

The 'Buffy the Vampire Slayer' spin-off graphic novel series, 'Fray', in which there has been no Slayer called for centuries, until Fray that is.

...and of course, Doctor Who!