Any organized and easily accessed collection of information, be it physical (a building full of books) or virtual (an on-line database of documents).
In computer programming, a library is a collection of standard functions and operations used by programming languages to execute certain tasks. The purpose of a library and the advantage of a well-stocked one is that it can significantly cut down the time required to develop a new computer application, as standard routines can simply be obtained from the library and combined with new codes, routines, etc., that the programmer creates. These new routines can also be added to the library, should they be required again.