Tagging - Who

You!
Tagging is an uncontrolled vocabulary that you choose to describe "stuff". This makes it a bottom-up vocabulary, instead of a top-down vocabulary like the Library Of Congress' subject headings.
Pros
You can decide what terms to use to describe things - terms that make sense to you.

Cons

Other people may use different terms, or misspelled terms, to describe the same "stuff".