Different types of light sources emanate different 'temperatures' of light. The colour of a subject depends not only on its own characteristics for reflecting and absorbing light, but also on the colour temperature of the light source illuminating it. The image below was taken under mixed light in a fishmarket. The right hand image has been color corrected.
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Daylight is defined as a mixture of direct sun and open blue sky illumination as found in Washington DC (USA) on a clear day between the hours of 10am and 4pm!
Clearly, not all pictures can be taken under standard conditions so digital cameras have to cope with different lighting.
Cameras see the colour temperature of light sources very differently to the way we do. Our brain adjusts automatically to relatively large differences in colour temperature without our conscious perception. Light sources need to be radically biased for us to notice them.
When the conditions change (e.g. under cloud or artificial lighting) most digital cameras set their white balance to match them automatically.
Technical
The temperature of an equivalent hot black body. A black body is an object which produces a smooth spectrum when heated (i.e. when glowing). The black body is equivalent to the light source in question in the sense that it produces the same relative amounts of red, green, and blue light. Color temperature is measured in Kelvin, K (degrees above absolute zero).