structural:expansion, quality:corruption
By size of the building

Apartment buildings are multi-story buildings where three or more residences are contained within one structure. Such a building may be called an apartment building, apartment complex, flat complex, block of flats, tower block, high-rise or, occasionally, mansion block (in British English), especially if it consists of many apartments for rent. A high-rise apartment building is commonly referred to as a residential tower, apartment tower, or block of flats in Australia.

A high-rise building is defined by its height differently in various jurisdictions. It may be only residential, in which case it might also be called a tower block, or it might include other functions such as hotels, offices, or shops. There is no clear difference between a tower block and a skyscraper, although a building with fifty or more stories is generally considered a skyscraper. High-rise buildings became possible with the invention of the elevator (lift) and cheaper, more abundant building materials. Their structural system usually is made of reinforced concrete and steel.

A low-rise building and mid-rise buildings have fewer storeys, but the limits are not always clear. Emporis defines a low-rise as "an enclosed structure below 35 metres which is divided into regular floor levels." The city of Toronto defines a mid-rise as a building between 4 and 12 stories.