A skillion roof is normally a single sloping roof surface, not attached to another roof surface. Skillion roofs are sometimes called a shed roof, a flat roof, in Australia or a lean-to in the UK.
The term skillion can also be used for a smaller addition to an existing roof, where keeping to the same slope (roof pitch) puts the skillion roof lower than the ceiling height of the main structure. In this case even though the main roof has a flat ceiling, the skillion part will have a sloping ceiling line to maximise the ceiling height.
Skillion roofs can also be used to provide clerestory windows for a hallway or similar room where a row of windows is placed below the edge of the skillion section reaching above the other roof below.