In fiction, time travel typically works in one of two ways. The first states that time is completely linear and singular in nature (i.e. the Terminators go back in time, kill John Conor; thus the human resistance never forms because he died before Skynet took over).
The other method ties in with the multiverse theory, which states that there is an entire universe for every possible outcome of an action. So, if you are at a crossroads, there is a universe where you went left and another where you went right. If you go back in time and change something, then that action begets another new universe (so you have your original universe and a new universe which feels the ramifications of your time travel). Most time travelers thus have a device included on their time machines that allows them to return to their proper universe as well as time period.
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