The first of the large film formats in photography. This film comes in fairly stiff sheets (the backing is thicker than in roll film), in order to minimize sagging in the film holder.
This film is usually shot in technical cameras or studio cameras.
Each sheet is notched on one of the short sides, to make individuation of the emulsion side easier.
The beauty of it: a large piece of film means a lot of resolution. Since it is sheet film it can be individually processed, so you can push and pull process every exposure. You never have to worry about which film is loaded in the camera body.
The horror of it: it is very expensive. A box of Velvia in 4"x5" (10 sheets) will cost maybe like 5 rolls of 36 exposures 35mm film. It is very error-prone: not for the casual user.
Film holders are fiddly objects, never entirely convenient. Not every film is available in this format.