A library is a collection of information, sources, resources, and services: it is organized for use and maintained by a public body, an institution, or a private individual. In the more traditional sense, a library is a collection of books.

This collection and services are used by people who choose not to — or cannot afford to — purchase an extensive collection themselves, who need material no individual can reasonably be expected to have, or who require professional assistance with their research.

However, with the collection of media other than books for storing information, many libraries are now also repositories and access points for maps, prints, or other documents and artworks on various storage media such as microform (microfilm/microfiche), audio tapes, CDs, LPs, cassettes, videotapes, and DVDs. Libraries may also provide public facilities to access CD-ROMs, subscription databases, and the Internet.

Thus, modern libraries are increasingly being redefined as places to get unrestricted access to information in many formats and from many sources. In addition to providing materials, they also provide the services of specialists, librarians, who are experts at finding and organizing information and at interpreting information needs.

More recently, libraries are understood as extending beyond the physical walls of a building, by including material accessible by electronic means, and by providing the assistance of librarians in navigating and analyzing tremendous amounts of knowledge with a variety of digital tools.

The term "library" has itself acquired a secondary meaning: "a collection of useful material for common use," and in this sense is used in fields such as computer science, mathematics and statistics, electronics and biology.

Organization

Libraries almost invariably contain long aisles with rows of books.

Libraries have materials arranged in a specified order according to a library classification system, so that items may be located quickly and collections may be browsed efficiently. Some libraries have additional galleries beyond the public ones, where reference materials are stored. These reference stacks may be open to selected members of the public. Others require patrons to submit a "stack request," which is a request for an assistant to retrieve the material from the closed stacks.

Larger libraries are often broken down into departments staffed by both paraprofessionals and professional librarians.

• Circulation handles user accounts and the loaning/returning and shelving of materials.
• Technical Services works behind the scenes cataloguing and processing new materials and deaccessioning weeded materials.
• Reference staffs a reference desk answering user questions (using structured reference interviews), instructing users, and developing library programming. Reference may be further broken down by user groups or materials; common collections are children's literature, young adult literature, and genealogy materials.
• Collection Development orders materials and maintains materials budgets.