What exactly are HRM's counting?

I only wear mine while working out and don't know tons about them myself but....an old friend of mine wore a fancy one on her arm 24/7 and it would sync with her computer and track all of her calories burned thoughout the day and then she would just enter in her food and it would give her calories in and calories burned for the entire day.

But as I mentioned, I only wear mine during my workout. I press start when I press start on my treadmill (or start doing my workout, whatever it may be) and then I press end when I get my HR back to resting when i'm finished.

HRM's are counting the beats per minute your heart completes in a given duration of time. That said, it is only an estimate since you could actually wear it watching a scary movie and have it show you burn 300 calories in an hour if your heart rate stayed that high through the whole movie. And we all know you didn't burn that much.

HRM's also don't correctly count calories in anaerobic (usually weight lifting) training. Your heart rate can go up lifting heavy weights but if you compare the same heart rate for cardio, the actually calories burned will be distinctly different.