What is .Net Framework Class Library (FCL)?

The .NET Framework class library is collections of namespaces, classes, interfaces & value types that provide access to system functionality. It is the foundation on which .NET Framework applications, components, and controls are built.

For Ex.:  In Every .Net Program we use system namespace

using System;

namespace CSharp
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Welcome to Dotnetpeoples...");
        }
    }
}

and this system namespace present in System assembly. 

And this system assembly located at path -> C:\Windows\Assembly (Which is also called as GAC)

Global Assembly Cache. So All the assembly which are reside in GAC are part of .Net Framework class library 

and get installed automatically along with .Net framework.