I don't really know why they have five different sites, because they're all the same, except for the name and color scheme. You can even register on one and then log in to any of the others with the same account.
Anyway, it uses Google Maps software to allow you to map your outdoor routes for, well, whatever it is you do. So if you walk or ride to work, or around your neighborhood, or anywhere that's along city streets, you can figure out how many miles you're going. You can also make your routes public or private and view other users' routes.
You can also use the site if you don't run, etc. outdoors, like, uh, me (in my defense, I was taking regular outdoor walks when I signed up). It has a really nice workout log function, where you can record any kind of workout (even weight training), and it will estimate your calories burned, keep a running total of your calories and mileage, and even track the mileage on your shoes. There's another area where you can record your daily calorie intake, but I don't think it has a full on diet log.
In short, I have found it pretty useful, and fairly user-friendly, so I thought I'd share.