Shutter speed represents the time that the shutter remains open when taking a photograph. It is measured in seconds or fractions of a second.
The total exposure of film is proportional to the duration and amount of light that reaches it.
Just as with aperture stops, shutter speeds can
be set in fractional stops.

Typical shutter speed scale:
1 1/2 1/4 1/8 1/15 1/30 1/60 1/125 1/250 1/500
Each of the numbers in this sequence (each stop) represents a halving to the right, or doubling to the left of the amount of light from its immediate neighbour.

If you want to avoid motion blur and achieve sharp images, assuming a normal camera and a steady hand, you should use a minimum shutter speed equal to 1 over the focal length of the lens i.e. a 28 mm lens can be held steady at 1/28 (1/30). If your shutter speed is slower than the reciprocal of the focal length of your lens, you should use a tripod to achieve a sharp image.

As a general rule you can’t achieve sharp shots at speeds slower than 1/60, but with rangefinder cameras (see Rangefinder Vs. SLR) because there is no vibration from mirror slap people say they can go down to 1/30 although this is below the reciprocal of a 45mm lens. Legend has it that Cartier Bresson could shoot hand held at f1.4 at 1/4, but as a rule stick to 1/60 and you’ll avoid unwanted motion blur.