Why is it called white noise? I know, that the white noise is called "white noise", because it comes from physics and has something to do with the spectral decomposition (is that right?) of the white light? I am not familiar with signal theory, so could anyone explain me why it is called white noise, especially the relation to the terms light, spectra and so?
 A: White noise is a signal (e.g., a sound or image) that has approximately equal power in every frequency band. In other words, its power spectral density (PSD) or power spectrum, is flat. (If you're unfamilar, the PSD/Power Spectrum/Spectrum is a plot showing the spectral content of a signal; that is, it shows the amount of power in/at each frequency or frequency band). People sometimes also use the term for sequence of uncorrelated random variables.
The White Noise page at wikipedia has several examples, if you want to see what it looks like, but the "ssssh" sound of turbulantly flowing air or the static on a detuned analog TV set are pretty reasonable approximations.
The name arises from an analogy with white light, which was thought to contain equal energy at all frequencies. This is actually not quite correct--it should be all wavelengths--but the name appears to have stuck and it's not clear to me how literal it was meant to begin with. The Oxford English dictionary reports that it was used as early as 1922, but it looks like the term didn't really catch on until the 1940s:

1922   Nature 1 Apr. 414/2: Just as the spectrum of a hot body normally consists of a continuous spectrum of white light, together with certain spectrum lines the wave~lengths of which are characteristic of the radiating material, so an element emitting X-rays not only gives out ‘white’ radiation, but superposes its characteristic lines on the general spectrum.
1943   Jrnl. Aeronaut. Sci. 10 129/1   Inside the plane it is different; there all frequencies added together at once are heard, producing a noise which is to sound what white light is to light... That white noise is annoying needs little argument.

In many, or probably even most contexts, white noise has little to do with light itself. There are other "coloured" noises which have different statistical properties. Red noise, for example, has a power spectrum dominated by low frequencies. As with white noise, the name may have been inspired by the fact that red light is at the low frequency end of the visible spectrum. However, a realization of (e.g.) red noise may not necessarily appear red, and a red-hued patch of noise may not be "red noise".
