A library is kind of container/list for creating, organizing and managing different types of documents/contents and share it with other team members. It allows to store files and meta information about the files so that it can be used among different team members. You can create, store and manage almost every type of file in different kind of libraries for example word documents, spreadsheets, presentations, forms etc. By default a Shared Library is created when you create a site and then on additionally you can create as many as type of libraries you want. This article talks abour different types of libraries and covers creating/managing document library in details, for more details click here.