Common Language Runtime (CLR): 

The heart of the .Net Framework.
CLR serves as the execution engine of .NET Framework.
All .NET programs execute under the supervision of CLR.
Common Language Runtime (CLR) provides a language-neutral platform for application development and execution and provides following services:

• Exception handling
• Garbage Collection
• Security
• Interoperability

.NET Framework Class Library (FCL):

The .NET Framework class library is a collection of classes, interfaces,optimize the development process and value types that provide access to system functionality. The .NET Framework class library is the foundation on which .NET Framework applications, components, and controls are built.

The .Net Framework class library (FCL) organized in a hierarchical tree structure and it is divided into logical grouping of types for the purpose of identification, is known as namespaces. The Classes are accessed by namespaces, which reside within Assemblies.