Whats an assembly?

Assemblies are the building blocks of the .NET framework.

which provides reusebility,security and fast accesible 
etc..once  created Assembly any where can use the assebly 
by calling the assembly name..

assembly is an collection of namespace

One of main thing in ASP NET is that the assembly is created by
default when we build our application or DLL. We can also check the
details of the manifest of the assembly by using classes located in the
System.Reflection namespace. We can create two types of assemblies in
ASP NET.

Private Assembly
Shared Assembly
There are many advantages of assembly some of them are as follows:
(1) Increased performance.
(2)Better code management and encapsulation.
(3)Introduces the n-tier concepts and business logic.
In asp dot net System namespace and other namespaces like it i.e. System.Data, System.Web are also pre-built assemblies that have been provided in the dot net framework to us by Microsoft.
 Differences Between Shared and Private Assemblies and how to use them.

Shared and Private Assembly: A private assembly is a assembly that is available
to particular applications where they are kept. And cannot be references outside
the scope of the folder where they are kept.
In contrast, Shared Assemblies are the assemblies that are accessible globally/shared 
across the machine to all the applications. For using the shared assemblies you need
to register the assembly with a strong name in the global assembly cache(GAC)
using gacutil.exe. GAC can be found on all the computers with .Net framework installed.