Physicists disagree on what the real possibilities might be, for time travel. Each of the theories has been explored several times in novels and movies alike.
  • Coherent History: Events can't be changed.
    If a traveler goes back, and adjusts something, he finds out later that his origin time hasn't changed at all; the adjustment was fated--nee required!--to happen.
  • Bi-modal History: Two histories cause each other.
    Terminator and Terminator 2: Judgement Day create the two histories that invent themselves. SkyNet sends a Terminator back to destroy John Connor and fails. John Connor grows up, leads the resistance against SkyNet, and sends a Terminator back to protect his child-self. The technology that leads to SkyNet comes solely from the parts of the first Terminator.
  • Free History: You can change anything.
    Usually, these plots also include the convenient resolution; if you change something important, all its ramifications show up. Timecop revolved around such manipulation. The young Senator is wounded; the old Senator grows an immediate scar. The Senator is killed, nobody but the time-traveler knew that History changed.