Your basal metabolic rate is the number of calories your body burns just by virtue of your being alive - breathing, dividing cells, pumping blood, growing fingernails, blinking, etc. You can calculate yours here.
At 25, 6'2", 177 lbs, my BMR is 1935.7 calories. What's yours (no need to list age/height/weight if you don't want to)?
Interestingly, as a blast-from-the-past post by Spice noted, fidgeters (like me) burn quite a few extra calories per day on top of this - in the neighborhood of 350 calories per day according to the NYT.
So I start by burning 1935.7, add 350 for relentless fidgeting, add another 908 for my daily activities, 100 for every mile I run, 250 for every half hour of swimming, and 750 for every hour of cycling - that's almost 3700 calories burned during a rougly average day! I can't imagine that I eat more than 2500, so why don't I weigh 110 lbs? Not that I want to, mind you - I'm just wondering which of these numbers is wrong.