They're tunnels through folded space. In theory, you could travel almost anywhere in a few minutes. And if you can control em, you could time travel. But not before the creation of the wormhole.
I don't know how the guys here plan to make one. Every book I read on the subject says the wormhole will collapse on you after you enter it. It would be required to hold the wormhole open with 'exotic matter', matter which consists of negative mass.
Now that I'm on the subject of wormholes, I've heard some people say that time dilation isn't time travel. Well what if you connected a wormhole, say, one end on earth and the other on a spaceship going near the speed of light.
You'd be able to travel through the wormhole to the future, and than back again. Isn't this time travel?