A space station is a man-made structure that is designed for people to live on. A space station is distinguished from other manned spacecraft by its lack of major propulsion or landing facilities - instead, other vehicles are used as transport to and from the station. Space stations are designed for medium-term living in orbit, for periods of some months.
Space stations are used to study the effects of long-term space flight on the human body as well as to provide platforms for greater number and lenght of scientific studies than available on other space vehicles.
Past and present space stations:
Some space station designs have been proposed which are intended as long-term space habitats for large numbers of people, essentially "cities in space" where people would make their homes. Thus far all such designs are only hypothetical, and have never been seriously considered for actual implementation.