Hi
The API is the programmer's interface to the Windows Operating system. With the API, you're programming with C or C++. You can go very deep into the system but that's hard and not always common to all Win32 platform.
MFC is a high-level encapsulation of the API. It provides a set of common classes needed for Windows programming: windows, menus, controls, system, ... You use only C++.
MFC should fit most of your needs, except if you plan to dig deep into Windows. It is much easier to use MFC as it takes care of all the gory details of the API.
With MFC, you concentrate your work on the functionality of your application. It's much more productive and your code should ( hum ....) migrate quite easily from one version of Windows to another ( if you follow the Microsoft rules, which is what Microsoft do not always do ...)
HTH
Thierry