Creating and using dll files in VC++ and its advantages?

How to create a dll file and more importantly how to use a dll file in the C++ code if i download some dll file..

The creation and use of DLLs with VC++ can be somewhat complicated depending on your requirements. If you create your own using the MFC wizard, it's not too bad at all. A great explanation with lots of code is provided in the CodeProject links below.

As for the advantages of DLLs:
The original purpose for DLLs was saving both disk space and memory required for Windows applications by sharing a single library between two loaded programs. DLLs provide the standard benefits of shared libraries, such as modularity. Another benefit of the modularity is the use of generic interfaces for plug-ins. A single interface may be developed which allows old as well as new modules to be integrated seamlessly at run-time into pre-existing applications, without any modification to the application itself. This concept of dynamic extensibility is taken to the extreme with ActiveX. Also, DLLs can be used as an indirect form of Inter-process communication. While the DLL code may be shared, the data is private except where shared data is explicitly requested. However, DLLs have a number of drawbacks, collectively called "DLL hell" (e.g. incompatible versions, DLL stomping, in-memory code pages).