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.

The 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.

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.