A WikiBadge? is a page which you add as a link to other pages to indicate some category or classification. It's just like a tag. When you follow such a link, you'll see the badge page which should describe it's meaning. Click the title on the badge page to see all the pages marked with that badge (its backlinks). WikiWikiWeb:WikiBadge has more about the concept, including pros and cons.

After page naming, wiki badges are the main way of categorising content in a traditional wiki. In zwikis, we have another organising tool: [page hierarchy]?. It's no problem to use both at once, just remember you can have only one hierarchy but as many badges/tags as you like.

We used to mark badge pages themselves by tagging them with the WikiBadge badge, but that has been linked in discussion too many times (could be cleaned up). Right now we identify badges by parenting them under this page. Here's an overview of this wiki's badges and badged pages (using DTML):