Use this tag for questions about software libraries. This tag is NOT for questions about computer programs used in public lending libraries or other brick-and-mortar libraries. Questions asking us to recommend or find a library are off-topic.
A library is a collection of non-volatile resources used by programs on a computer, often to develop software.
Libraries contain code and data that provide services to independent programs. This encourages the sharing and changing of code and data in a modular fashion, and eases the distribution of the code and data. Library files are not executable programs. They are either static libraries that are merged with an executable when the executable is being compiled and linked, making them "statically linked", or they are dynamic libraries that are loaded by a dynamic linker while the executable is running, making them "dynamically linked". The dynamic linker may also allow an application to explicitly request that a module be loaded and to obtain references to routines in the module; this can be used to implement plug-ins.
The above was taken from the Wikipedia entry Library (computing).
Questions asking us to recommend or find a library, documentation or other off-site resource are off-topic for Stack Overflow as they tend to attract opinionated answers and spam.