different light sources have different spectra. there is "warm" light, "cool" light, whatever.
If you look with your eyes at a scene illuminated by a certain type of light (e.g. tungsten light), then your brain compensates to some extent and you "see" sort of normal colours.
However, if you take the image with a camera, and look at it later, then the image is taken out of context and your brain does a less good job in compensating. You appear to see false colours.
Hence the photographer uses filters to compensate the type of light while shooting. or, when shooting digital, he can set the white balance in his camera (in such a way that white is white without a colour cast).
white balance is in particular important when it comes to skin tones.