A lot of digital cameras these days (not just the expensive ones) let you fiddle with ISO settings and juggling these carefully can really help your photos in darker areas or places where you can't use, or don't want to use, flash to get a more natural shot.
From the early days of film the ISO standard has been used to give a rating for light sensitivity.
The upshot of this is easy, higher ISO means your camera is more sensitive to light. This means your camera's "open" for less time which means less blur.
For example I've just grabbed random pic in the room to see the difference between ISO, shutter speed and apeture...
|
ISO
|
100
|
200
|
400
|
800
|
1600
|
|
Shutter
|
1"
|
0"6'
|
0"3'
|
1/8th"
|
1/13th"
|
|
Apeture
|
f4
|
f4
|
f4
|
f4
|
f4
|
As you can see, as ISO doubles, shutter speed approximately halves.
On the negative side, on digital cameras a higher ISO means more 'noise' and graining on the snap.
Most cameras have a default of ISO 100 with various steps up to around ISO 3200 for the REALLY fancy cameras, most will end a lot shorter than that, but it all makes a difference.
As an example I took five pictures of a Family Guy DVD cover, with the image stabilisation on my lens switched off and the camera set to auto. I then cropped out Cleveland. All these pics are 100% sized (when clicked on) and haven't been edited in any way.
ISO 100 - 2 and a half second exposure
ISO 200 - 1 second exposure
ISO 400 - half a second exposure
ISO 800 - quarter of a second exposure
ISO 1600 - 1/8th of a second exposure
As you can see, the shutter speed shrinks and the picture gets sharper.
However, there is a flipside. If you look at the ISO 1600 pic it seems very grainy compared to the others, in this situation probably an ISO 800 shot is the best compromise, sharp enough, especially if you're making the image smaller, but with a lot less 'noise'.
It's a balancing act really, but if it's darker and you're finding the pictures are blurring finding and changing your ISO setting can help, sometimes you're happier with a sharper shot than a less 'noisy' one.
Just remember to put them back before you go shooting in good light 
Usksider
Pro 
Just a thought (and feel free to delete this) I just wondered if it'd help to put a table here iso/stop/shutter... on the premiss that all three are connected.