Hi! So what is a wormhole?
Wormhole is kind of similar to black hole.
It does have really strong gravitational field but not that much as black hole. The thing is that black hole bends space time much more than wormhole — it has singularity in the middle. Wormhole does NOT have singularity, because it does not have enough strong gravity.
When it does not have singularity it also can not have event horizont.
So what makes it a hole?
When it bends spacetime it can create tunnel from one point of universe to another through higher dimension this could be used for traveling really long distances. Black hole cannot do this because its gravity is too strong.
Now it is clear so lets move to spacetraveling through wormholes. First you can imagine it with lower dimensions which are pretty easy. So 0th dimension is pretty useless since it is just point so we will jump right for 1st dimension. It is only line, not very complicated right? You have line which 1km long. You have to get from one end to another but you dont have enough time. You will take shortcut through 2nd dimension! You will take this line and bend it to circle, so now that point which you wanted to visit is right next to you! From 2nd dimension to 3rd it is also very easy to imagine, you can actually see it above. Take a paper (that is your space) and on one edge draw point and draw same one on opposite part. Then bend this paper so those points are touching, you have been traveling faster than light! Well it is not fair, it´s like saying Usain Bolt to run 1000m and you running only 50m, quess who will win, light.
It is named wormhole because worm in apple is using 3rd dimension instead of traveling on the surface of apple.
Problem with wormholes is that higher dimensions than 4th (which is time) are only in really small scale: 10^-33 centimeters. This length is not really usable for traveling but really old wormholes could be bigger caused to expanding of universe. Another problem is their stability because they would need to have lot of “exotic” matter to last longer than moments. Last thing is that wormholes are only hypothetical, wormhole has never been observed.
PS: I have edited it once because there were some mistakes and it did not make any sense before.
PSS (26.11.2015): light is not 4th dimension as you can read in this post.