It's a website that you can track your fill-ups, and there is an option to create a signature badge that you can put in your signature on forums and stuff.
You can also look at the real-world MPGs other people are getting with various vehicles.
I only use it as a back-up to the big excel spreadsheet I keep on my computer, and the app I have on my phone to record my fill-ups.
2013 WK2 Overland HEMI 4x4
Bright White / New Saddle