In a discussion about creating deliberate slow exposure the discussion got around to ISO. I put in this answer (slightly edited for here) and I thought my answer might be useful more widely so hence this post.
ISO is the measure of sensitivity of the sensor in your camera. The lower the ISO the better the quality of the image.
The acceptable ISO settings are quite camera dependant and can be subjective. Some will give little or no noise up to 6400 iso. As a general guide here is what I use:
Landscape on a tripod 50 - 100 iso
Landscape hand held 100 - 400 iso
Street daytime bright 100 iso
Street dull day 400 iso
Street evening whatever it takes - I can shoot at night with 6400 with acceptable hand held results
Stop action (eg. bike racing) what ever ISO it takes to get the fast shutter and long dof that I want
Panning action 100 iso generally
In old school photography with film (aka classic) we were taught the relationship between shutter and aperture but rarely discussed ISO because we couldn't change it (except with advanced dark room techniques). Today we need to think of shutter aperture and ISO as part of a balanced triangle because we can change the ISO whenever we want.
When you change one of the tips of the triangle it affects the other tips and your image. The higher the ISO the less light you need to activate the sensor to create an image but the more noise (unattractive colour or brightness changes from pixel to pixel). If you need a faster shutter to stop the action and have a sharp image at an acceptable depth of field then the only element you can change to achieve that is the ISO.
As an example, during the some street photography I was photographing a dancer at night, I wanted sharp, she was moving quickly and I wanted a short dof - I wanted to see the crowd watching her but I wanted attention on her. I didn't mind a bit of noise and I didn't want to distract her with flash. I needed at least 1/20th to hand hold with acceptable sharpness and I wanted f/1.8 for very low depth of field. I changed the ISO until my light meter indicated an acceptable exposure at ISO of 500 and took the shot. Well a series, never rely on just one! Between the first few I checked my outcome and made minor adjustments. I was actually very lucky with this. 1/20th is nowhere near what I usually need for a hand held with sharp results - as a general rule of thumb for my and my shaky hands I double the focal length so in this case I should use 170 - I braced my body against a light pole to minimise my movement.