Since the beginnings of .NET, the desktop and console applications have been bootstrapped by executable files and the traditional ASP.NET applications are bootstrapped by IIS using an ISAPI DLL. The applications written in any language supported by .NET are compiled to an assembly. An assembly is an EXE or DLL file containing Intermediate Language (IL). This IL file needs to be compiled to native code as the operating systems and CPUs don't understand IL, and this is called just-in-time (JIT) compiling.

JIT compiles the IL code to native code just before it executes on the machine where the assemblies are deployed. The JIT functionality is part of the .NET CLR or common language runtime.

The CLR is responsible for loading assemblies, checking types, and garbage collection. So, it is necessary to install the .NET Framework on the machine application runs. The big stack of classes and other types are available. It contains all types needed for Windows Forms, WCF, WPF, web forms, and types usable across these frameworks, such as file handling, reading, and manipulating XML, drawing, and cryptography. All applications use some of these classes.

The CLR is specifically designed to run on Windows. Also, some of the classes in the FCL are specifically for Windows. System.web is an assembly containing classes that are tied to IIS and therefore Windows. building blocks of legacy .NET Framework are as follows: