Well, this is really a vast topic. I will try to give you a glimpse of it so that as a beginner it does not become a nightmare for you at start.
What is .Net Framework?
In object-oriented, a framework is a set of classes (abstract classes) and interfaces related to a particular domain. So .Net Framework is set of classes related to (business) applications. Structure of the building is made (more than 50%). Now you have to fill it out. Some boundaries are decided you have to be within it.
How .Net Framework works?CLS (Common Language Specification)
If your language for example, FORTRAN comply with CLS, will comply with .Net e.g. FORTRAN.Net. It will use all libraries that C# is using.
CLR (Common Language Runtime)
When C#.Net code is compiled using CSharp compiler then it is converted to MSIL. Now to execute MSIL, CLR is used. Note that MSIL is like byte-code in Java and CLR is like JVM of Java.
a. Class Loader
Its purpose is to load classes.b. Code Verifier
Its duty is to verify the MSIL (Microsoft Intermediate Language) is in the form that it can be executed.c. Garbage Collector
Memory management is done automatically by this. That is why “.Net” code is called “managed code”. To explicitly call it we use method GC.Collect();d. Runtime Type Check (RTC)
It is used for late binding, polymorphism etc.e. Security Manager (SM)
It is to check for example logged in person’s security that either he has rights or not.f. COM Marsheller
It is used for using COM and DCOM components. It is for backward-compatibilty.