One of the major difference between a consumer digital camera and a digital Single Lens Reflex (DSLR) is that the former produces images with a lot of noise when using high ISOs and long exposure times, and the latter is practically noise-free (though high ISO performance varies depending on camera manufacturer and model). Noise is apparent by the presence of color speckles where there should be none. For example, instead of a blue sky, you notice faint pink, purple and other color speckles amongst the otherwise blue sky.
"Image noise" is the digital equivalent of film grain for analogue cameras. Alternatively, one can think of it as analogous to the subtle background hiss you may hear from your audio system at full volume. For digital images, this noise appears as random speckles on an otherwise smooth surface and can significantly degrade image quality.
Camera Noise or Grain refers to the amount of distortion to your images that is caused as a result of using certain ISO / ASA settings. It can also be the result of enlarging images bigger than the tolerance of the film or digital settings within your camera will allow.
Would you like to learn how to avoid Noise? Keep close! ;)