Assemblies are block of code in .net. there are three types of assemblies are there is .net. (1)Private Assembly (2)Public Assembly (3)Satellite Assembly Private Assembly is used by only one application that calls it. Public Assembly is put on shared folder of applications and then can be used by the multiple application at the same time. Satellite Assembly is culture specific. you have to create different assembly in different language and the correct one will be loaded as local culture.

In the Microsoft .NET framework an assembly is a partially compiled code library for use in deployment, versioning and security. In the Microsoft Windows implementation of .NET, an assembly is a PE (portable executable) file.

The code in an assembly is compiled into MSIL, which is then compiled into machine language at runtime by the CLR.

An assembly can consist of one or more files. Code files are called modules. An assembly can contain more than one code module and since it is possible to use different languages to create code modules this means that it is technically possible to use several different languages to create an assembly.

You can find more assembly faqs at .Net assemblies Faqs

Thanks
Gurjinder Singh Brar
http://www.dotnetinterviewfaqs.com/

If we defined assembly in this way then what about PIA? This could be one of them. Tell me if i'm wrong....;)

Assembly is  a reusable,versionable,self describing bulding block of a CLR.
3 types of Assemblies :
1.Pivate Assembly:is local to the perticular folder.
2.Shared Assembly:copy of assembly to set of applications through out system.
3.Sattellite Assembly:An assembly containing localized resources for another assembly.

An assembly is a core part of the runtime. An assembly is the collection of all information required by the runtime to execute your application. This information is referred to as the Metadata. An assembly can be a DLL file or a Portable Executable (PE) file. The Common Language Runtime (CLR) can only execute code in assemblies and the assembly must include a manifest or reference a manifest contained in another file.