Film plates is a modern circumscription of dry plates which are a variant of photographic plates. They have to be distinguished from wet plates which are simply glass plates which fit into the plate holder of a wet plate camera. Those glass plates have to be prepared in a dark chamber with a light sensitive emulsion and must be exposed when still wet. The first of these processes was the Wet-collodion process.
Dry plates are also glass plates or better glass sheets, but coated with a light sensitive emulsion in the factory. The emulsion film on the glass plate is dried before the plates are packaged light-tight for distribution. The Albumen plates were an early variant of these film plates. For most dry plate cameras the plate holders have to be loaded in a dark room. But then the plates in the light-tight holders can be used much later, making the photographer independent from a near dark room or from carrying a dark tent.
Dry Plates were in available in several standardized plate sizes.