Multi-family residential (also known as Multi-dwelling unit or MDU) is a classification of housing where multiple separate housing units for residential (i.e. non-commercial) inhabitants are contained within one building. A common form is an apartment building. Sometimes units in a multi-family residential building are condominiums, where typically the units are owned individually rather than leased from a single apartment building owner.
Two-flat - a building basically like a house, commonly built on a house lot, consisting of a flat (apartment) taking up the first floor, another flat taking up the second floor, usually a common basement, a common front entrance, foyer, and stairs to the second floor, and often a similar back entrance, foyer, and stairs. In old buildings, the back entrance, foyer, and stairs may have been added on later. Sometimes there are front verandas for each of flats, one above the other. Typically the whole building is owned by the same party.
Three-flat - a building similar to a two-flat except there are three flats. Two-flat and possibly three-flat buildings are rather common in certain older neighborhoods in certain cities.
Four-flat - a building similar to a three-flat except there are four flats. In some cases, the arrangement of apartments may be different and the lot size may be larger than that of a regular house.
Duplex or semi-detached- One building consisting of two separate "houses", typically side by side, each with separate entrances and typically without common inside areas. Each of the two houses typically has separate owners.
Townhouse - a house attached to any number of other townhouses each of which may have multiple floors, commonly side by side each with their own separate entrances. Each such house has its own owner.
Apartment building - a building with multiple apartments. There can be multiple apartments on each floor and there are often multiple floors. Apartment building can range in many sizes, some with only a few apartments, other with hundreds of apartments on many floors, or any size in between. There are often inside hallways and inside entrances to each apartment, but outside entrances to each apartment are also possible. An apartment building can be owned by one party and each of the apartments rented to tenants or each of the apartments can be owned as a condominium by separate parties.
Mixed use building - a building with space for both commercial, business, or office use, and space for residential use. Possible arrangements include the commercial/business use on the first or first couple floors and one or more apartments or residential spaces on the upper floors. Another possibility is to have the commercial/business area up front and the residential area in the back. Some or maybe all of the space may be used by the owner or some or all the business and residential units may be leased by the owner. Condominium ownership is also possible.