Film masks offer a very cost-effective path to photolithography. If your features are several tens of microns across, then this is certainly an option for you. If you area doing soft lithography with channels 80-100um across, then film masks are an efficient means to an end.
Although commonly referred to as transparency masks, film masks are not "transparencies", at least not the kind I used to generate with a laser printer shortly before each presentation. Film masks are essentially a large sheet of photographic film that have been exposed with light from a plotter (hence the other common name: photoplots). After exposure, the film is developed, fixed, and dried in a dark room.
Whereas a hard mask typically consists of chrome-on-glass, a film masks consist of emulsion-on-polyester. The emulsion is a mixture of silver-halide salts suspended in a colloidal material such as gelatin. The emulsion offers much higher optical density than what you would get from spewing toner out of a laser printer.
Still with me? Then you should read the following well-written article describing the history and technology of emulsion in much greater depth.