It's a product from Line6 that uses the same amp modeling thechnology as the POD I use to record with. In addition it plugs onto the computer and the internet and gives you access to sites that allow you to jam with original artists' backing tracks with the guitar removed.
I don't know all the pros and cons of it, but there's a discussion board at line6 where you can find out first hand about it all.
Go
here to find out more o the Line 6 forums.