Halloween celebrations most probably go back to Celtic harvest festivals, when guises and lanterns were used to scare evil spirits away. In the US and the UK (and in several other related countries), people carve pumpkins into scary faces to put next to their doors with a candle inside to ward off evil. Children go “trick or treating” in the afternoon or in the evening: they put on (supposedly) scary costumes, visit each home in the neighborhood, and ask for a “toll” at each door. The residents may decide whether they pay the toll (the treat, usually in the form of various candies), or suffer the consequences (usually in the form of a prank, like getting their car wrapped in toilet paper). People often throw costume parties this night.
In Hungary, we do not have any special traditions connected to Halloween. While carving pumpkins and having costume parties is gaining popularity, trick or treating is definitely not something you should expect. If you take your children trick or treating, most people answering the doors will be baffled at what you might want from them.
At the same time, you can usually find a few events for children, organized by expat groups, so they can enjoy the traditional Halloween feeling even if they cannot really go trick or treating.