Folksonomy is a classification system in which end users apply public tags to online items, typically to make those items easier for themselves or others to find later. Over time, this can give rise to a classification system based on those tags and how often they are applied or searched for, in contrast to a taxonomic classification designed by the owners of the content and specified when it is published. This practice is also known as collaborative tagging, social classification, social indexing, and social tagging. Folksonomy was originally "the result of personal free tagging of information [...] for one's own retrieval", but online sharing and interaction expanded it into collaborative forms. Social tagging is the application of tags in an open online environment where the tags of other users are available to others. Collaborative tagging (also known as group tagging) is tagging performed by a group of users. This type of folksonomy is commonly used in cooperative and collaborative projects such as research, content repositories, and social bookmarking.

The term was coined by Thomas Vander Wal in 2004 as a portmanteau of folk and taxonomy. Folksonomies became popular as part of social software applications such as social bookmarking and photograph annotation that enable users to collectively classify and find information via shared tags. Some websites include tag clouds as a way to visualize tags in a folksonomy.

Folksonomies can be used for K–12 education, business, and higher education. More specifically, folksonomies may be implemented for social bookmarking, teacher resource repositories, e-learning systems, collaborative learning, collaborative research, professional development and teaching. Wikipedia is a prime example of folksonomy.^{[better source needed][clarification needed]}

Folksonomies are a trade-off between traditional centralized classification and no classification at all, and have several advantages:

There are several disadvantages with the use of tags and folksonomies as well, and some of the advantages can lead to problems. For example, the simplicity in tagging can result in poorly applied tags. Further, while controlled vocabularies are exclusionary by nature, tags are often ambiguous and overly personalized. Users apply tags to documents in many different ways and tagging systems also often lack mechanisms for handling synonyms, acronyms and homonyms, and they also often lack mechanisms for handling spelling variations such as misspellings, singular/plural form, conjugated and compound words. Some tagging systems do not support tags consisting of multiple words, resulting in tags like "viewfrommywindow". Sometimes users choose specialized tags or tags without meaning to others.

A folksonomy emerges when users tag content or information, such as web pages, photos, videos, podcasts, tweets, scientific papers and others. Strohmaier et al. elaborate the concept: the term "tagging" refers to a "voluntary activity of users who are annotating resources with term-so-called 'tags' – freely chosen from an unbounded and uncontrolled vocabulary". Others explain tags as an unstructured textual label or keywords, and that they appear as a simple form of metadata.

Folksonomies consist of three basic entities: users, tags, and resources. Users create tags to mark resources such as: web pages, photos, videos, and podcasts. These tags are used to manage, categorize and summarize online content. This collaborative tagging system also uses these tags as a way to index information, facilitate searches and navigate resources. Folksonomy also includes a set of URLs that are used to identify resources that have been referred to by users of different websites. These systems also include category schemes that have the ability to organize tags at different levels of granularity.

Vander Wal identifies two types of folksonomy: broad and narrow. A broad folksonomy arises when multiple users can apply the same tag to an item, providing information about which tags are the most popular. A narrow folksonomy occurs when users, typically fewer in number and often including the item's creator, tag an item with tags that can each be applied only once. While both broad and narrow folksonomies enable the searchability of content by adding an associated word or phrase to an object, a broad folksonomy allows for sorting based on the popularity of each tag, as well as the tracking of emerging trends in tag usage and developing vocabularies.