I've noticed a number of different versions of time travel as has been given by various fiction authors in various universes.
Type 1 time travel - Timelines are preset. The universe is self-repeating and anything that happens via time travel is already predestined in the universe. In the way Trixsie described, events caused by time travel happened all along and time is unchanged. (example: Back to the future)
Type 2 time travel - Actions done in the past or future eventually affect what has happened, aside from the tangled timeline of time itself, there exists a constant timeline which always travels forward that determines the flow of time for time travelers. However the flow of time in the regular timeline is disrupted and often altered through quite subtle actions and often-serious consequences, something science fiction authors rarely touch on. In such a timeline, any interaction with the past universe whatsoever will cause it to alter, which is inevitable.
There are a number of subtypes but these are the two main ones. The other versions are less describable and more obscure, so I'll just leave them to a later time when I'm more *ahem* awake.