The term "apartment" is favored in North America, whereas the term "flat" is sometimes, but not exclusively, used in the United Kingdom and most other English-speaking areas and Commonwealth nationsFact: date=September 2007. In Malaysian English, "flat" often denotes a housing block of lesser quality meant for lower-income groups, while "apartment" is more generic and may also include luxury condominiums.

Some apartment-dwellers own their own apartments, either as co-ops, in which the residents own shares of a corporation that owns the building or development; or in condominiums, whose residents own their apartments and share ownership of the public spaces. Most apartments are in buildings designed for the purpose, but large older houses are sometimes divided into apartments. The word apartment connotes a residential unit or section in a building. In some locations, particularly the United States, the word denotes a rental unit owned by the building owner, and is not typically used for a condominium.

The word unit is a more general term referring to both apartments and rental business suites. The word is generally used only in the context of a specific building. E.g., "This building has three units" or "I'm going to rent a unit in this building", but not "I'm going to rent a unit somewhere."

When there is no tenant occupying an apartment, the lessor is said to have a vacancy. For apartment lessors, each vacancy represents a loss of income from rent-paying tenants for the time the apartment is vacant (i.e., unoccupied). Lessors' objectives are often to minimize the vacancy rate for their units. The owner of the apartment typically when transferring possession to the occupant(s) gives him/her the key to the apartment entrance door(s) and any other keys needed to live there, such as a common key to the building or any other common areas, and an individual unit mailbox key. When the occupant(s) move out, these keys are typically returned to the owner.

Apartment types and characteristics

Moving up from the efficiencies are one-bedroom apartments where one bedroom is a separate room from the rest of the apartment. Then there are two-bedroom, three-bedroom, etc. apartments. Small apartments often have only one entrance/exit.

Large apartments often have two entrances/exits, perhaps a door in the front and another in the back. Depending on the building design, the entrance/exit doors may be directly to the outside or to a common area inside, such as a hallway. Depending on location, apartments may be available for rent furnished with furniture or unfurnished into which a tenant usually moves in with their own furniture. A garden apartment has some characteristics of a townhouse: each apartment has its own entrance, and apartments are not placed vertically over one another. However, a garden apartment is usually only one story high and never more than two stories; they are often one-bedrooms and almost never more than two-bedrooms. Some garden apartment buildings place a one-car garage under each apartment, with pedestrian entrances from a common courtyard open at one end. The grounds are more landscaped than for other modestly scaled apartments. (Alternately, "garden apartment" can refer to a unit built half below grade, putting its windows at garden level.Fact: date=February 2008}