In combination with variation of the lens aperture, this regulates how exposed the film will be. A fast shutter speed demands a larger aperture to avoid under-exposure, just as a slow shutter speed is offset by a very small aperture to avoid over-exposure.
Shutter speed is measured in seconds. A typical shutter speed for photographs taken in sunlight is 1/125th of a second. Very short shutter speeds are used to freeze fast-moving subjects, for example at sporting events. Very long shutter speeds are used in low-light conditions such as night or to intentionally blur a moving subject for artistic effect.
In early days of photography, available shutter speeds were somewhat ad hoc, but later a standardised 2:1 scale was adopted, which can be extended at either end:
The rough guide used by most 35mm photographers is that the slowest possible shutter speed that can be used with great care is the shutter speed numerically closest to the lens focal length.
For example, for handheld use of a 35mm camera with a 50mm normal lens, the closest shutter speed is 1/60 s. Note that using this with "great care" would normally mean bracing the camera, arms, or body to minimise camera movement. For a free standing, unsupported photographer it is usually necessary to use the next fastest shutter speed which would be 1/125 s in this case.
Other 35mm hand held examples are:
28mm wide angle lens, 1/30 s may be used with care, and 1/60 s is advised.
105 medium telephoto lens, 1/125 s may be used with care, and 1/250 s is advised.
300mm long telephoto lens, 1/250 s may be used with care, and 1/500 s is advised.
In cinematography, shutter speed is a function of the frame rate and shutter angle. Most motion picture film cameras use a rotating shutter with a shutter angle of 170° to 180°, which leaves the film exposed for about 1/48 or 1/50 second at standard 24 fps speed.