There is a short but pretty good article what "calories" over on Yahoo's health section.
It myth-busts on a few of my favorite things about calories, like, what are they (they're the heat generated by burning the food in a lab, basically), they aren't created equal (calories can't be traded one-for-one), and so on.
The Truth About Calories
It's a quick read and it's pretty much on target. It's why your macronutrient ratios matter, why your overall intake matters but maybe not the way you think, and what calories even are in the first place.