What does a "CALLBACK" declaration in C do?

The "CALLBACK" is a calling convention. There are other kinds of calling conventions. CALLBACK is the same as __stdcall.

http://www.codeguru.com/cpp/cpp/cpp_mfc/callbacks/article.php/c10557

Some more information at Raymond Chen's blog:

http://blogs.msdn.com/oldnewthing/archive/2004/01/08/48616.aspx

Short roundup from Raymond Chen's Blog:

The great thing about calling conventions on the x86 platform is that there are so many to choose from!

C calling convention (__cdecl)

The C calling convention is constrained because it allows the use of functions with a variable number of parameters. It pretty much requires that the stack be caller-cleaned and that the parameters be pushed right to left, so that the first parameter is at a fixed position relative to the top of the stack. In summary: Caller cleans the stack, parameters pushed right to left.

Pascal calling convention (__pascal)

Pascal does not support functions with a variable number of parameters, so it can use the callee-clean convention. Parameters are pushed from left to right. Nearly all Win16 functions are exported as Pascal calling convention. The callee-clean convention saves three bytes at each call point, with a fixed overhead of two bytes per function. It was also fractionally faster. On Win16, saving a few hundred bytes and a few cycles was a big deal. Note: The Fortran calling convention (__fortran) is the same as the Pascal calling convention

It's the calling convention. It's required when you pass a pointer to this function to a Windows API which later calls that function. The Windows calling convention is different from the C calling convention, therefore you need to specify to the compiler that WndProc() is special, and that it needs different startup and cleanup code.