A wormhole, also known as an Einstein-Rosen Bridge, is a hypothetical topological feature of spacetime that is essentially a "shortcut" from one point in the universe to another point in the universe, allowing travel between them that is faster than it would take light to make the journey through normal space. The name "wormhole" comes from an analogy that was used to explain this; imagine that the universe is the skin of an apple, and a worm is travelling over its surface. The distance from one side of the apple to the other is equal to half the apple's circumference if the worm stays on the apple's surface, but if it instead burrows a wormhole directly through the apple the distance it has to travel is considerably less.
Exotic matter, which has negative energy density, may be required to maintain a wormhole.
A wormhole could potentially allow time travel. This could be accomplished by accelerating one end of the wormhole relative to the other, and then sometime later bringing it back; relativistic time dilation would result in less time having passed for the accelerated wormhole mouth compared to the stationary one, meaning that anything which entered the stationary wormhole mouth would exit the accelerated one at a point in time prior to its entry. Such a wormhole pair is sometimes referred to as a "timehole." It is thought that it may not be possible to convert a wormhole into a time machine in this manner, however; some mathematical models indicate that a feedback loop of virtual particles would circulate through the timehole with ever-increasing intensity, destroying it before any information could be passed through it. This has been disproved, however, in that, in a "timehole," general amounts of radiation would disperse after traveling through the wormhole, therefore preventing infinite accumulation. This is as described by Kip S. Thorne from the book, Black Holes and Time Warps. 
See also: Spacecraft propulsion