Hi Priyanka12,
.dll is Data Link Library, is nothing but physical separation of code., Whenever you build your project it its compiled into MicroSoft Intermediate Language so called MSIL., Its programming language indipendent.
You can use this ddl than onwards to any other project where-ever its functionality can be implemented, reused with any language i.e dll created based on C# is not limited to reuse only for C# projects you can use it with any .Net supported OOP language. For this better example is AJAX Control Toolkit you can use this dll in C# base project as well as VB.Net (and like) based project without any difference.
Thanks and Regards,
Rk_Hirpara