A folksonomy is a system of classification derived from the practice and method of collaboratively creating and translating tags to annotate and categorize content; this practice is also known as collaborative tagging, social classification, social indexing, and social tagging. Folksonomy, a term coined by Thomas Vander Wal, is a portmanteau of folk and taxonomy. Vander Wal explains some of the characteristics of folksonomies by identifying two types: broad and narrow. A broad folksonomy is the one in which multiple users tag particular content with a variety of terms from a variety of vocabularies, thus creating a greater amount of metadata for that content. A narrow folksonomy, on the other hand, occurs when a few users, primarily the content creator, tag an object with a limited number of terms. While both broad and narrow folksonomies enable the searchability of content by adding textual description - or access points - to an object, a narrow folksonomy does not have the same benefits as a broad folksonomy, which allows for the tracking of emerging trends in tag usage and developing vocabularies.  Folksonomies became popular on the Web around 2004 as part of social software applications such as social bookmarking and photograph annotation. Tagging, which is one of the defining characteristics of Web 2.0 services, allows users to collectively classify and find information. Some websites include tag clouds as a way to visualize tags in a folksonomy. However, tag clouds visualize only the vocabulary but not the structure of folksonomies, as do tag graphs.
An empirical analysis of the complex dynamics of tagging systems, published in 2007, has shown that consensus around stable distributions and shared vocabularies does emerge, even in the absence of a central controlled vocabulary. For content to be searchable, it should be categorized and grouped. While this was believed to require commonly agreed on sets of content describing tags (much like keywords of a journal article), recent research has found that, in large folksonomies, common structures also emerge on the level of categorizations. Accordingly, it is possible to devise mathematical models of collaborative tagging that allow for translating from personal tag vocabularies (personomies) to the vocabulary shared by most users.
The term folksonomy is generally attributed to Thomas Vander Wal. It is a portmanteau of the words folk (or folks) and taxonomy that specifically refers to subject indexing systems created within Internet communities. Folksonomy has little to do with taxonomy — the latter refers to an ontological, hierarchical way of categorizing, while folksonomy establishes categories (each tag is a category) that are theoretically "equal" to each other (i.e., there is no hierarchy, or parent-child relation between different tags).
Folksonomy is a type of collaborative tagging system in which the classification of data is done by users. Folksonomies consist of three basic entities: users, tags, and resource. 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 by users of different websites. These systems also include category schemes that have the ability to organize tags at different levels of granularity. 
There are two different groups of folksonomies. There are broad folksonomies which have many users contributing to the creation of tags and narrow folksonomies where only a few users are tagging particular items. A broad folksonomy allows many people to tag the same resources and any user can tag a resource using their own vocabulary. In a narrow folksonomy, only a few people are able to create tags and these tags are used by other users to locate resources. Unlike broad folksonomies, narrow folksonomies are not very common. An example of a broad folksonomy is del.icio.us, this is a website where users can tag any online resource they find relevant with their own personal tags. An example of a narrow folksonomies can be found in systems used by large businesses; these types of folksonomy are mainly used for research and associates working together in collaborative groups.
Folksonomy is unrelated to folk taxonomy, a cultural practice that has been widely documented in anthropological and folkloristic work. Folk taxonomies are culturally supplied, intergenerationally transmitted, and relatively stable classification systems that people in a given culture use to make sense of the entire world around them (not just the Internet).
Folksonomy may hold the key to developing a Semantic Web, in which every Web page contains machine-readable metadata that describes its content. Such metadata would dramatically improve the precision (the percentage of relevant documents) in search engine retrieval lists. However, it is difficult to see how the large and varied community of Web page authors could be persuaded to add metadata to their pages in a consistent, reliable way; web authors who wish to do so experience high entry costs because metadata systems are time-consuming to learn and use. For this reason, few Web authors make use of the simple Dublin Core metadata standard, even though the use of Dublin Core meta-tags could increase their pages' prominence in search engine retrieval lists. In contrast to more formalized, top-down classifications using controlled vocabularies, folksonomy is a distributed classification system with low entry costs. The Insemtives project is investigating methods of motivating users to contribute semantic content.
Some libraries are adding tagging features into their online public access catalog, or OPACs, in addition to use of standardized subject headings, in order to encourage a more social, participatory, or Web 2.0 nature to the catalog. While this empowers users to contribute to an otherwise closed cataloging system, it can only supplement and not completely replace traditional cataloging.
The study of the structuring or classification of folksonomy is termed folksontology. This branch of ontology deals with the intersection between highly structured taxonomies or hierarchies and loosely structured folksonomy, asking what best features can be taken by both for a system of classification. The strength of flat-tagging schemes is their ability to relate one item to others like it. Folksonomy allows large disparate groups of users to collaboratively label massive, dynamic information systems. The strength of taxonomies are their browsability: users can easily start from more generalized knowledge and target their queries towards more specific and detailed knowledge. Folksonomy looks to categorize tags and thus create browsable spaces of information that are easy to maintain and expand.
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- SocialTagging.org provides short definitions of key terms related to tagging and folksonomies
- Vanderwal's definition of folksonomy
- Vanderwal's take on Wikipedia's definition of folksonomy