Here's the way I've always heard time travel theorized:

Plastic Time: It's very easy to alter the timeline, but you run the risk of drastically changing the world--or even yourself--by accident. Go back in time and kill Hitler as a baby? Fine, World War II never happens, but now your grandparents never met in that London hospital...

High-Resistance Plastic Time: It's very hard to alter the timeline. Any changes you make will probably not be noticed. Go back in time and shoot Lincoln in Ford's Theater? Fine, but John Wilkes Booth still gets blamed...

Fixed Time: You can't alter the timeline, no matter what. Go back in time and kill Hitler as a baby? Whoops, you picked the wrong house. Try to nuke ancient Egypt? Too bad the bomb's a dud...

Chaotic Time: The timeline is extremely easy to alter, and any changes you make will propagate wildly. Go back to the 1950s and buy a newspaper? Now France rules the world. Go back to the Permian Age and step on an ant? The human race no longer exists... (See Ray Bradbury's short story "A Sound of Thunder" or the "Time and Punishment" episode of "The Simpsons" for some good examples of this)

Paradox-Proof Time: Very similar to Fixed Time, but if you start to do anything that would alter the timeline, you immediately get bounced back to your own time where you can't do any damage. Alternately, you can change history, but it's always the history of a parallel universe--never your own. So you can kill Hitler in his crib, but WWII still takes place in your universe.