Tags

remouk

No, the main purpose of tags is to let one discussion have multiple tags, instead of a single category.

Not sure it's the main purpose: labels or multi-category are already fine for this. Tags are keywords defined by users. Btw, I said "one of" the main purpose, of course there are many other possible applications.

remouk

Here is a cool implementation of tags in a forum software. Any registered user can add a tag on any conversation, while the category system still exists.