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1 Cal as it is most often used in food industry is in fact 1 kcal, which is 1000 cal or 4184 Joules. So a food calorie (Cal) is the energy required to raise 1kg of water by 1 Kelvin (or Celsius, doesn't make any difference in this case).
Regarding the calories of individual component, Carbs do in general give you about 4 Cal and so do fats. Proteins give you 9 Cals.
However, I remember reading somewhere recently, that you cannot judge the actual energy you obtain by these values for few reasons. First reason was mentioned by Rodney. The other one is that the more complex the molecule gets the less energy (per gram) your organism will be able to get from it even if it manages to break it down completelly. This is due to the fact that during every step of the breakdown (which increase with the complexity of the molecule), a small fraction of heat is lost and therefore is not used to make ATP.
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If you multiply that by infinity and take it to the depths of forever, you will, perhaps, get just a glimpse of what I am talking about. --Meet Joe Black--
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