Much has been made of digital noise being similar to grain in film. And like film grain, the photographer needs to decide to use it or lose it, often depending on the image. For example, if the image is a landscape meant for large format reproduction, then noise is a bad thing, with less being better. On the other hand, if the image is a street scene or a music show, then noise can add a gritty, gutsy feeling, one that is lost by eliminating all the noise. Even if you choose to keep the noise it helps to know what it is and what causes it to occur. With a little knowledge you can manipulate noise to your advantage, either accentuating or minimizing its appearance.
It may help to know that noise is not unique to digital photography, but a side effect that occurs with all electronic devices, for example, the background hiss of a radio, or the distorted sound of an over-amplified guitar. In many cases it is accentuated by heat. Heat caused by the rapid movement of electrons across a circuit, and ambient heat from physical surroundings. Both of these have an adverse effect on the quality of digital images and the appearance of noise.
Something else to consider: as the camera sensor ages, noise becomes more pronounced. This means that unlike film cameras, digital cameras should be replaced every few years. A general rule of thumb for high-end cameras is after every 100,000 actuations (shutter releases).
In digital photography although there are several types of noise, three types are the most prevalent. These are color, luminance and hot pixels. Major camera makers address this noise with some degree of success, particularly in high-end cameras.