Lots of people ask me what camera 'noise' is.
Noise is like grain on a film photograph – unsightly little speckles on your image. With so many people using digital cameras, it just as well that you know that can affect you.
The higher the ISO the more noise will appear and the more you enlarge the image the more you can see it. Night time images are prone to noise as the camera struggles to record detail. To reduce noise, use the largest image setting for the best quality.
Always use a tripod so that you can choose the lowest ISO setting without causing blur - normally ISO 100 or 200 is ideal!
Hope this helps!