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A tag works like a keyword. Like categories, they are used to describe people, groups, or posts. You can tag a post with several tags separated by commas.

Example: tags for the post your wrote about your holiday in Australia could be: holiday, australia, down under, sun, hot, melbourne, sydney, dingos.
Many tags help to have your posts be found more easily, but fewer tags also help you with organizing your posts.

However, most users tend to use tags for their own personal organisation of posts and do not allow other people to tag their posts.