Hi,

Assembly is nothing but Building block of .net framework Applications .
For Example: you can create  any Application that Appliccation when ever build that will be created EXEor DLL file based on the templetes .you vcan take Console Application that will create EXE file,Othewise you can take ClassLibray templete that will be created DLL file.
Assemblies are Clssified into Two types
1.Private Assembly.
2.Pubilc or Shared Assembly.
->private Assembly is an assembly that can be used single .Net Application.By default Assembly is Private Assembly.why it can be prafed means it is provide Security.
->Pubilc Assembly is an assembly that can be used more than one .net Application, by using Add refferece, it can Stroed in GAC.

Thanks,
Narasima