Book excerpt: Collaborative information services on Web 2.0 are used by Internet users to produce digital information resources, and to furnish the contents of the resources with their own keywords, so-called tags. This book deals with collaborative information services and folksonomies as a method of representing knowledge and a tool for information retrieval. Collaborative information services on Web 2.0 are used by Internet users not only to produce digital information resources, but also to furnish the contents of the resources with their own keywords, so-called tags. Whilst doing so the user is not required to comply with rules, as is necessary with a library catalogue. The amount of user-generated tags in a collaborative information service is referred to as folksonomy. Folksonomies allow users to relocate their own resources and to search for other resources. This book deals with collaborative information services and folksonomies both as a method of representing knowledge and a tool for information retrieval.