Ultra low floor is a term describing vehicles such as trams and buses whose passenger compartment has a floor which is considerably lower than that of traditional cars. A recent development in the transport industry, vehicles of this type have a stepless entry and an area without seating next to at least one of the doors where wheelchairs and perambulators can be parked.
The low floor may extend over the complete or just part of the length of the car. If extended over the full length, there is no place for bogies or even axles connecting corresponding left and right wheels (they would be at a higher level than the floor); this is solved with single-wheel drives: motors integrated into the wheels. 
In addition, in buses a hydraulic "kneeling device" can be used when the car is not in motion, tilting it to one side and thus lowering it even further towards the surface level of the road. This technology enables less mobile passengers to board and leave the vehicle without help from others.
Ultra low floor trams and buses are currently being further developed and are already in use in various parts of the world, generally in urban areas.
An ultra low floor tram (ULF) in Vienna, Austria (July 2003)