Framework Class Library known as FCL in .Net Framework
FCL is a collection of reusable objects and components of different types including interfaces , classes, and data types in .NET Framework, that provide access to core functionality of system.
.Net Framework Class Library (FCL) is the foundation or base on which we develop application further, it provides a ready to use controls, classes, UI etc, those are re-usable for different type of application like WCF application, Console Applications, Thick client applications, ASP.NET applications, Web services, service oriented applications etc.
.NET Class Library is integrated with the CLR (Common Language Runtime) of the Framework, which manages the code execution. These are basically building blocks , can be extended further.