Social bookmarking sites provide a way to organize, collate and categorize the web using a classification structure called “tags.”
Tags are little more than labels that are applied to a given, bookmarked resource. Tagging is a bottom-up, user-generated form of categorization that allows people to use whatever vocabulary is helpful to them. While this may seem a bit chaotic and messy, it has become a common method to annotate and classify content on the web.
Article source: https://www.seo-online.website