As has been said above, the light is controlled by a computer, which basically "guesses" for lack of a better term, how much life is left in your oil. If you don't reset the light when you change your oil, the computer still thinks your running the old stuff.
The yellow light just means that service is due. It'll go out when you reset the indicator. If you ignore it long enough, the red light will come on. That just means that you ignored the yellow light.
BTW, next time it'll say that your car is due for inspection, as the inspection light comes on during every other oil change.
hth
Brian