Noise means electronic noise, generated within the sensor and associated electronics. Noise is usually greater at higher ambient temperatures and is more prevalent in smaller sensors, particularly the compact cameras, which even though some pack 7+ Mp have unnacceptably high noise at high ISO )(the equivalent of film ASA) e.g. 200 and above settings. Canon have managed to get their noise levels particularly low especially in the latest versions, that's the main reason for their popularity. The 350D and 20D/30D and upwards are all useable even at 1600 ISO. If you are wanting to compare cameras and look at some reviews and sample images try
http://www.dcresource.com/reviews/cameraList.php