Shutter speed refers to exposure time. Basically how long it takes the camera to capture the image! In low light conditions such as night, you'll want your shutter speed open as much as possible to allow as much of the available light to get in. That's why you would want a tripod, because the longer the camera takes to take the photo, the more still you have to be. If you hand hold the camera, it will end up blurry. If you're in a well lit area, you won't need a long shutter speed because there will be enough light.

ISO determines the film's sensitivity to light (if using a film camera) / sensitivty to the image sensor based on how much light is getting to the camera.

The ISO therefore ends up affecting your shutterspeed. THey go hand in hand. If you're in a low light condition and you have a fast shutterspeed that you would use in daylight, you can bump your ISO up to 800 and it will have your photo well lit, you don't have to use a tripod or anything. The downside is that your photo will be grainy. Where as if you had a LOW ISO such as 100, you would have to have a longer shutterspeed to compensate for the lack of light getting into the camera.

Here, at a concert I used a high ISO and a fast shutterspeed because I obviously couldn't afford a slow shutter speed otherwise my photos would have been blurry! In focus photos but grainy :-(

one (http://i279.photobucket.com/albums/kk136/nadineada/concerts/lady%20gaga%20200310/a91fe3f5.jpg) & two (http://i279.photobucket.com/albums/kk136/nadineada/concerts/lady%20gaga%20200310/ba25f54f.jpg)

I hope that helped D: