You know when you go to a Facebook Fan Page for a company, and they have a banner that pops up that says "LIKE us and join the party!", you click "Like", and the banner changes to something that says "Thank you!" or similar? That banner is created on a tab, which is just part of the page. It's called a welcome-reveal tab. They're only important for Fan Pages, really, and now that Facebook has changed lay outs, they aren't really tabs anymore, per say.