It is hard to think of a design pattern that is more democratic than the tag. Tagging allows a user to categorize content through keywords that they define. This is in direct opposition to the traditional approach of a site creator/admin erecting an organization structure and then authorizing content to its divisions. It is primarily seen on apps that focus on user generated content like Facebook, Tumblr, Pinterest, etc.
Tags are usually placed in close proximity to the content it is categorizing. Each tag turns into a link after it is added, which leads to a listing of all the content submitted under that specific keyword. This helps content consumers discover new categories.