Black holes? and time travel

Time can most simply be thought of as another dimension like the 3 spatial dimensions. If I were to observe an event at a point in space, A today and then a separate event also at point A tomorrow. The only way to determine these distinct events it to add a fourth co-ordinate to 3D space, time. Instead of thinking of a point in space using 3 co-ordinates relativity uses a 4 dimensional co-ordinate system, x = (ct, x, y, z).
For a good description of picturing 4 dimensions with a 3D mind read Hyperspace by Michio Kaku, while im not sure about the final conclusions of the book it has an excellent description of how to think about 4 dimensional systems (and indeed higher if you are into such theories).