Wormholes

To put it simply, a wormhole is a shortcut in spacetime.

» How it works

Imagine that you are a cute little worm on an apple. You burrow through the centre of the apple and then end up on the other side. So, instead of going the long way across the surface of the apple, you have made a shortcut through the apple!

Now, replace the surface of the apple with 4D spacetime. The wormhole that was just created is now a shortcut between a point in the past and a point in the future, and the wormhole itself is a fold in a higher dimension.

» How we can control it for our own diabolical purposes
(hypothetically, of course)

Say you have a ready-made wormhole with both ends in a reasonable distance from yourself (relativly). Now just obtain control of a large mass (Jupiter looks nice) and use that as bait. Hover it around the wormhole until the wormhole is attracted to your bait, in this case, Jupiter's large gravitational field.

Now accelerate both objects (somehow) to near the speed of light. At these speeds, time dilation should take place. Now just keep on doing that until the time between the other end of the wormhole and your end is how much you want it to be. Now take your end of the wormhole back to its original place in space (but it won't be in its original point in time! Ha ha!).

Congratulations! You have just created a shortcut between one point in time and another!