Simple explanation from me is, the Zune Card is to Zune what the Xbox Tag is to Xbox. Plain and Simple. The Zune card shows your last played songs, etc and the Xbox 360 tag shows last played games, etc...

 

This is my Zune Card above...

Signing up for the Zune Social is easy but using it is much more fun with a little understanding of what is happening behind the scenes. Your Zune Card can display all the tracks and artists you've been listening too. The Zune software sends the tracks you've played to your Zune Card every 15 mintues. So don't worry if after playing a few songs, you don't instantly see them on your Zune Card. But what about songs that you listen to on your Zune? Those are transmitted soon after you sync your device.

If your tracks aren't showing up on your Zune Card, even after 15 min, it is probably a good idea to make sure you are logged into your account in the Zune software. Also, tracks that you stream from within the marketplace won't show on your Zune Card.

Read more over at I Want a Pink Zune.