This answer expands on the answer given by hdhondt
Interestingly, there are strong parallels between the trade-offs involved in photography and with electronic sensors.
From the wikipedia article about silver halide:
"When a silver halide crystal is exposed to light, a sensitivity speck on the surface of the crystal is turned into a small speck of metallic silver (these comprise the invisible or latent image). If the speck of silver contains approximately four or more atoms, it is rendered developable - meaning that it can undergo development which turns the entire crystal into metallic silver. Areas of the emulsion receiving larger amounts of light (reflected from a subject being photographed, for example) undergo the greatest development and therefore results in the highest optical density."
Photograpnic film can be made in a range of different sensitivities.
As explained in the wikipedia article: the latent image consists of extremely local spots (just a couple of atoms), where silver atoms have been reconstituted to an unbound state. The chemical process of development is that conditions are created such that those local spots of reconstituted silver induce neighbouring bound silver to reconstitute too. (the conditions are heat and manipulating the acidity of the gel.)
For low light conditions high sensitivity film is used. The silver halide crystals of high sensitivity film have been created in such a way that a single spot (of latent image) will on development induce reconstitution of a comparatively large area.
Conversely, low sensitivity film is such that a single reconstituted spot will on development induce reconstitution of a comparatively small area.
The trade-off: the sensitivity of the photographic film sets an upper limit to the resolution of the developed image. An image photographed with a high sensitivity film is grainier than an image with a low sensitivity film.