Hmmm, I've never even heard of it. Only ever needed my passport. But then, as I'm already an EU citizen I guess the passport would be all I need.

I got curious and googled it! Sounds like it's just a way of combining all the participating countries' visitor visas into one. (Although I have no idea which countries doing this Shengen thing you'd have otherwise needed individual visas for, what with the whole EU citizen thing...)

Hooray I was somewhat unhelpful :D