Patio Doors (also known as sliding doors) slide past each other, one behind the other and so provide extra floor space than regular swinging doors.
Patio doors are fitted into tracks, one at the bottom and one at the top. These tracks are usually hidden with some kind of decorative trim and the doors simply hang between the two tracks, with rollers used to allow the doors to slide from side to side.
The only problem with sliding doors is that only approximately 50% of the space can be open at any one time, as when the doors are open they are still in part blocking the opening space you have. Sliding doors can be beneficial with restricted space, either externally or internally, as they do not protrude from the line of you wall when open. This is a definite benefit in restricted areas!