A simple little lightproof
box of plastic
than holds either a single sheet of film
for a view camera
or a roll of film for a medium format
For a single sheet holder, this box is barely larger than the film itself and really contains nothing save the dark slide and enough space inside to hold the film.
Roll film holders are more complicated - included within is the mechanism necessary to advance the film, the dark slide, a removeable holder within the holder that the film winds around and a counter to tell you when you're done.
While technological advances within roll film holders keep surging forward with anything from automatic film advancement to a self compacting dark slide to a lightproof holder within the holder, single sheet holders are virtually identical to their ancestors from the days when cameras were invented.