A high-rise building is a tall building. It is used as an apartment building, office building or other functions, such as hotel or retail. Sometimes, multiple purposes are combined. A residential high-rise building is also called a tower block or an "MDU", standing for "Multi Dwelling Unit". A very tall high-rise building is called a skyscraper.
High-rise buildings became possible with the invention of the elevator (lift) and cheaper and more abundant building materials. The materials used for high-rise buildings are reinforced concrete and steel. Most North American style skyscrapers have a steel frame, while residential blocks are usually constructed of concrete. There is no clear difference between a tower block and a skyscraper but a building with fifty or more stories is generally considered a skyscraper. The opposite of high-rise is a low-rise building and in the middle are mid-rise buildings.