A sliding door is a type of door that opens by sliding (usually horizontally), whereby the door is either mounted on or suspended from a track. These aren't usually used in small vehicles, but generally they're most commonly used for minibuses and buses to provide a large entrance or exit for passengers without obstructing the pavement.
They are often used on the side of commercial vans as well, as this allows a large opening for equipment to be loaded and unloaded without obstructing access.
Sliding doors are often used in mini MPVs such as the Toyota Porte and Peugeot 1007 and Renault Kangoo, but are more commonly used in full-sized MPVs like the Toyota Previa, the Citroën C8, the Peugeot 807, the Chrysler Voyager and the Kia Sedona. Their use has increased over the years as MPVs have increased in popularity, because it gives easy access and makes parking in tight spaces possible