All roofs, even ones that look flat, need to slope to some degree so that snowmelt and rainfall can drain off. But beyond that basic requirement, architects and builders have a lot of leeway, and they've used that creative freedom to invent a wonderful array of roof designs.
Shed roofs, shown on the three dormers of this house, slant in only one direction. They fall within the basic category of gabled roofs, provided you think of them as half-gables. Shed roofs are easy to build because all rafters are identical.