Regular dlls wraps only the c/c++ functions. Where extention dlls include c++ interfaces where we can create the objects of it and use in our classes. Extended dlls support object oriented concepts.Regural dlls uses mfc internally and exported functions can be used by any mfc or non-mfc applications.Extention dlls implements reusable classes derived from mfc library,built using dll version of mfc.Only mfc executables(applications/dll-shared version of mfc) can use extention dlls.extention dlls used for passing mfc derived objects b/w applications and dlls. Regulardlls linked both statically and dynamically but extended dlls links dynamically.