A library is a reusable set of code which software developer can use as it comes from its creators. Library usually focuses on a narrow set of functionality, which user accesses through an API. It performs specific, well-defined operations. User calls a library function, it executes some code and then control is returned to developer's code. Usually, it does not provide any hooks for developer to extend or override its behavior.
- Software library is a handy tool, reusable set of code, which software developer can use in his application to solve particular, well-defined problem.