3 types of DLL 

1.SDK DLL:Old type
2.Extension DLL: These DLL’s exposes variables and  
functions to the client application. It can expose C++ 
functions and C++ classes to be used in your application.

3.Regular DLL: MFC extension DLL’s can be used only with 
MFC applications. If you want a dll to be used by a wider 
range of applications then use a regular DLL.

Two Types
1. Regular DLLs : These DLLs can be used by C++ and other
language applications also. These DLLs will expose the
function in 'C' style.

2. Extension DLLs: Exclusively for C++ client. These DLLs
will expose the functions in 'C++' name mangling technique.
These symbols can be understood by only C++ client.

Two types of DLL:

1. Extenxsion DLl: These Dll only used in MFC application. 
these dll are not derived from CWinApp class. By this Dll 
we can export the function and classes both. In this Dll 
DllMain function is there for intialisation 

2. Regular Dll : These Dll used in MFC and non-MFc both 
type of applications. This is derived from CwinApp class. 
We can export function in this dll.

1. Extension DLL
2. Regular DLL

we can mention Resouce only dll also