when you register it you put in your height and weight and it calculates your stride and calories burnt on that info.
Pick up a couple of extra batteries, they last 5 or 6 months. Fitbit sent me a note yesterday saying mine was low. Rob's got a little flashing battery icon when his got low.
The first time I put mine on I ran up and down the driveway to see how the fitbit and the website worked. As I went tearing down the drive way my brand new first time meeting her neighbor was driving by and she screeched to a stop, thought I was having an emergency
On the other hand I'm pretty pumped by the fact at 62 I Can run!