An apartment is a self-reliant accommodation unit that occupies only part of a building. Apartments may be owned by an owner-occupier or rented.

Some apartment-dwellers own their 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. Apartment building owners or managers often use the more general word units to refer to apartments. Units can be used to refer to rental business suites as well as residential apartments. 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 transfers possession to the occupant(s) by giving him/her the key to the apartment entrance door(s) and any other keys need 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 should typically be returned to the owner.

Apartment types and characteristics
Apartments can be classified into several types. Studio or efficiency apartments tend to be the smallest apartments with the cheapest rents in a given area. These kinds of apartment usually consist mainly of a large room which is the living, dining, and bedroom combined. There are usually kitchen facilities as part of this central room, but the bathroom is its own smaller separate room. 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 the location, apartments may be available for rent furnished with furniture or unfurnished into which a tenant usually moves in with his own furniture. Permanent carpeting is often included in an apartment.

Laundry facilities are usually kept in a separate area accessible to all the tenants in the building. Depending on when the building was built and the design of the building, utilities such as water, heating, and electricity may be common for all the apartments in the building or separate for each apartment and billed separately to each tenant. Outlets for connection to telephones are typically included in apartments. Telephone service is optional and is practically always billed separately from the rent payments. Cable television and similar amenities are extra also. Parking space(s), air conditioner, and extra storage space may or may not be included in an apartment. Rental leases often limit the maximum number of people who can reside in each apartment.

On or around the ground floor of the apartment building, a series of mailboxes are typically kept in a location accessible to the public and, thus, to the mailman too. Every unit typically gets its own mailbox with individual keys to it. Some very large apartment buildings with a full-time staff may take mail from the mailman and provide mail-sorting service. Near the mailboxes or some other location accessible by outsiders, there may be a buzzer (equivalent to a doorbell) for each individual unit. In smaller apartment buildings such as two- or three-flats, or even four-flats, garbage is often disposed of in trash containers similar to those used at houses. In larger buildings, garbage is often collected in a common trash bin or dumpster. For cleanliness or minimizing noise, many lessors will place restrictions on tenants regarding keeping pets in an apartment.

In some parts of the world, the word apartment is used generally to refer to a new purpose-built self-contained residential unit in a building, whereas the word flat means a converted self-contained unit in an older building.

When part of a house is converted for the ostensible use of a landlord's family member, the unit may be known as an in-law apartment or granny flat, though these (sometimes illegally) created units are often occupied by ordinary renters rather than family members.