The term "Library 2.0" was coined by Michael Casey in his blog, LibraryCrunch, in 2005, although the concepts that collectively make up "Library 2.0" existed well before then.

Library 2.0 has a loose definition. One of the proposed definitions is: “Library 2.0 describes a subset of library services designed to meet user needs caused by the direct and peripheral effects of Web 2.0 services”.

In their famous article "Do Libraries Matter", Ken Chad and Paul Miller, executive director and technology evangelist of Talis, describe their view of the four principles of library 2.0:

• The library is everywhere
  - Available at the point of need
  - Go where the users are
• The library has no barriers
  - No barriers between users and information
  - At the hear of "democratization of information"
  -Open up your content to comments, reviews, tagging, rating and sharing
• The library invites participation
  - blogs, wikis, tagging and commenting in the catalog, etc...
• The library uses Flexible best-of-breed systems
  - ILS made of small pieces loosely joined.
  - Modular, Interoperable Web Services
  - Tailor services to users' needs (Chad & Miller, 2005).

As described by Maness (2006), the theory for Library 2.0 could be summarized by these four elements:

• It is user-centered. Users participate in the creation of the content and services they view within the library's web-presence, OPAC, etc. The consumption and creation of content is dynamic, and thus the roles of librarian and user are not always clear.
• It provides a multi-media experience. Both the collections and services of Library 2.0 contain video and audio components. While this is not often cited as a function of Library 2.0, it is here suggested that it should be.
• It is socially rich. The library's web-presence includes users' presences. There are both synchronous (e.g. IM) and asynchronous (e.g. wikis) ways for users to communicate with one another and with librarians.
• It is communally innovative. This is perhaps the single most important aspect of Library 2.0. It rests on the foundation of libraries as a community service, but understands that as communities change, libraries must not only change with them, they must allow users to change the library. It seeks to continually change its services, to find new ways to allow communities, not just individuals to seek, find, and utilize information. (p.2)

• Library 2.0 (L2) is a transformation in the way library services are delivered to library users.
• It provides new tools to make the library space (both virtual and physical) more interactive, collaborative and driven by community needs.
• It encourages collaborative two-way social interactions between library staff and library customers. 
• L2 requires user participation and feedback in the development and maintenance of library services.

Some of the familiar names connected to Library 2.0 are: Michael Casey, Michael Stephens, John Blyberg and Jenny Levine

Here are some links to check how a library 2.0 could look like:

• Ann Arbor District Library: http://www.aadl.org/
• Hennepin County Public Library: http://www.hclib.org/
• North Carolina States University, Libraries Catalog: http://www.lib.ncsu.edu/
• Memorial Hall Library: http://www.lib.ncsu.edu/
• Darien Library: http://www.darienlibrary.org/

Blogs:

• West Long Branch Public Library: http://www.wlbpl.org/
• Lansing Public Library: http://lansinglibraryweb.blogspot.com/
• Pembroke Library: http://pembrokeaccess.blogspot.com/
• Dublin City Libraries: http://dublinreaders.wordpress.com/

Wikis:

• Saint Joseph County Public Library: http://sjcpl.lib.in.us/subjectguides/index.php/Main_Page
• Summer Reading Club: http://summerreadingclub.pbwiki.com/

RSS:

• Dublin City Public Libraries: http://www.bloglines.com/public/dublincitypubliclibraries

Mashups:

• Allen County Public Library: http://blog.acpl.lib.in.us/amzamash/book_wall.php
• BiblioPage: Data is gathered from a variety of library records. The interface combines book records with Amazon data: http://www.bibliopage.com/

Instant Messaging:

• Thomas Ford Memorial Library: http://www.fordlibrary.org/chat/