A housewarming party is a special party to be held when someone buys or moves into a new apartment or house. The person or people who bought the property or moved are the ones who throw the party. The party is an opportunity for friends and family to congratulate the person on the new home. It also gives people a chance to see what the new home looks like. It is an opportunity to fill the new space with love and hopefully presents.
It is traditional to bring a gift to a housewarming party. Some people register a list of items they want or need for their new home at a local store or stores. Some common items people will put on a gift registry include kitchen tools like knives and items like curtains. Even if there isn't a registry, a good housewarming gift is something to decorate the new house with like a piece of art or a plant. You can also bring food or drinks to share with the other guests. This is often appreciated since at a housewarming there isn't a lot of food served, usually just appetizers or sandwiches. There are usually no planned activities like games at a house warming party. The host or hostess of the party will, however, probably give all the guests a tour of their new home. Sometimes, because a housewarming party happens shortly after a person moves into their new home, people may be asked to help unpack boxes. This isn't usual though.
Housewarming parties get their name from the fact that a long time ago people would actually bring firewood to a new home as a gift. This was so that the person could keep their home warm for the winter. Now most homes have central heating and don't use fires to keep warm.