A DLL is a dynamic link library. It provide a way of sharing common code at runtime . Any appication( exe) can load a dll in to memory at runtime, retrieving the address of the fuctions to be called / accessing resources or variables. client can use the functions, variable or resources and unloads the dll from memory.
One can load the dlls either statically or dynamically.
Dynamically loading -
HANDLE h = LoadLibrary("MyDll.dll"); // load dll
typedef FUNPTR (void) (*funcPtr ) (); // create a fuction pointer with matching signature // with that of exported function from dll.
FUNPTR show = GetProcAddress( h, "Show"); // Get function address
show(); // Call the fuction
FreeLibrary(h); // unload dll from memory
Include the .h file and specify .lib file to the linker for specified dll. User can access the dll content ( Exported fuctions, resources, variables) at compile time itself.
Regular, Extension and pure Win32 DLL (without MFC)
Regular DLL's can export only functions, variables or resources. These can be loaded by any Win32 environment (e.g. VB 5) Regular DLLs are generally larger in size. When you build a regular DLL, you may choose a static link (in this case MFC library code is copied to your DLL) and dynamic (in this case you would need MFC DLLs to be presented on the target machine)
Along with functions, variables and resources Extension DLL's can export Classes as well. It supports a C++ interface, i.e. can export whole C++ classes and the client may construct objects from them. Extension DLL dynamically links to MFC DLLs (those which name starts with MFC??.DLL) Extension DLL is usually small (simple extension DLL might be around 10K)
Pure Win32 DLL
This is simple dll, it does not use any MFC classes.