An Alternate Timeline is a subgenre of science fiction, it is set in a world, which History has been altered from actual history. Many science fiction franchises uses Time Travel as a plot device to travel to the past or to the future. This can be done intentionally, or unintentionally. In some science fiction time travel can be used on a regular basis, where a reliable method of time travel is possible, like in Doctor Who or Quantum Leap; in other science fiction there is no reliable means of time travel, like in Star Trek or Stargate, when time travel is used, the user can usually end up in a random point in time.
In some science fiction, people use time travel to go back in time to simply observe past events. On Star Trek: Enterprise during a time when there is a reliable method of time travel, people would go back observe how the Pyramids were made, but in Star Trek there is a Temporal Prime Directive, which is used when someone intentionally, or unintentionally travels back in time. There other those who would use time travel to go back and change history, some would do this for good, other for evil. Once again on Star Trek: Enterprise, the Enterprise NX-01 is caught up in a Temporal Cold War, where an alien race called the Suliban would receive information from the future. In Quantum Leap, Doctor Samuel Becket used the Quantum Leap Accelerator to travel to random points within his own life time, he did this to change history for the better, history was only changed in a small way. In the movie Stargate: Continuum, a Goa'uld System Lord called Ba'al developed a reliable method of time travel using the Stargate to go back in time and changed history on Earth, which affected the entire galaxy in the process, SG-1 was able to go back and change history again.