LRESULT (long) what does it do? How does it work in a program?

I learned that the LRESULT is the return value. LRESULT is a long, and can be written as long __stdcall fnWndProc(...). But what is a 'long' and how does it help the program...in this case I have a Windows Application interface and wondered what the LRESULT (long) did to help create the window or why it's there.
Thanks --