Both words refer to a kind of residence which is not a house with a garden but they are smaller residences. The first difference between them is that flat is a British English word, while apartment is apartment is an American English word. Although, apartment exists in British English, too but it has a different meaning: it refers to a set of large rooms with expensive furniture and decoration. Castles can be referred to as apartments or some particular public building, for example, the Royal Apartments. It also means a room furniture and set aside for a particular person. In this sense, the word is often used in plural form.
