To put it simply, a library can be defined as a building that stores various types of books.

 

The more definitive definition is this:

A library is a collection of resources in a variety of formats that is organized by information professionals or other experts who provide convenient physical, digital, bibliographic, or intellectual access and offer targeted services and programs with the mission of educating, informing, or entertaining a variety of audiences and the goal of stimulating individual learning and advancing society as a whole.

Definition courtesy from: ALA American Library Association