The Jack-'o-Lantern tradition actually started in Ireland, and the legend varies slightly depending on who is telling it, but it goes something like this.
In the 18th century and drunken, foul mouthed, miser named Stingy Jack invited the devil to go for a drink with him. The devil apparently agreed, and all was well until the bill arrived, which each thought the other should pay.
Jack somehow convinced the devil to turn himself into a coin in order to pay the bill, and the devil went along with the idea. However, instead of paying the bill, Jack put the coin into his pocket alongside a cross, effectively trapping the devil.
For reasons best known to himself, Jack decided to do the decent (but stupid) thing. He let the devil out after making him promise that the devil wouldn't come after Jack for one to ten years (this is where the legend differs depending who is telling it). The devil had to agree as he didn't want to spend eternity in Jack's pocket, and off he went.
At the end of the agreed time, the devil returned to Jack for his payback. However, Jack tricked the devil into climbing a tree to get an apple for their journey to hell. While the devil was getting the apple, Jack carved a cross into the bark of the tree, trapping the devil again!
Jack must have had a soft spot for the devil because he agreed to let him come down as long as he agreed never to take him to hell. The devil once more had to agree, and all was well.
When Jack died, he was rejected by St Peter because he was such an evil person, and the devil couldn't accept him into hell because of their agreement. So Jack was given a piece of burning coal by the devil, to light his path through purgatory. Jack carried this inside a hollowed out turnip.
Irish folk started to put carved out turnips on their windowsills on halloween to keep out Stingy Jack and other bad spirits. Some carved scary faces in them to scare anything bad away.
When Irish people started to move to the United States, they realised that the pumpkin, a native fruit there, would be ideal for carving. That is why you see Jack-o-'lanterns in windows all over America on Halloween.
However, it doesn't end there because so popular were the pumpkins that they began to be sold in the UK too. So now, pumpkins are in Europe for Halloween too, though they are rarely eaten here.
