Sliding glass doors, also called gliding patio doors, slide open and closed along upper and lower tracks. They are most often seen in private homes leading to patios or balconies. Because they come in two, three, or four panels, they look like large (sometimes panoramic) windows that just happen to serve as entrances as well.
Sliding glass doors are good for small areas where a swinging door would impose on space. Many do come with swinging doors as well, usually the inner two in four panel styles. They can also be used as sliding windows in larger rooms.