Let the contributers of information on your website add keywords (tags) to the content they submit. These keywords are then transformed into links that leads to tag pages; listing all other contributions for that tag (category).
Tags are relevant keywords associated with or assigned to a piece of information. Tags are often used on social websites, where users can upload their own content. Here, tags are used to let users organize and categorize their own data in the public sphere. In this way, tags can be seen as a bottom-up categorization of data rather than a top-down categorization of data, where the creators of the site define the hierarchy data is submitted to.
Tags added to a blog post.
When adding tags at flickr, you can choose from tags you gave other pictures (grayed out) or type new. Upon submission, each tag is shown above the input field in a vertical list. They can each be easily removed separately.
When adding tags to a video at Vimeo, tags are separated by commas, and each tag are upon submission added to a horizontal line below the input field. Each tag can easily be removed separately.
Tags at youtube.com