Search notes:

Windows CRT: tchar.h

tchar.h contains definitions for generic international text functions.

_tmain and _tWinMain

Depending on the value of _UNICODE, tchar.h defines _tmain and _tWinMain etc: 
#ifdef _UNICODE

   #define  _tmain       wmain
   #define  _tWinMain    wWinMain
   #define  _tenviron   _wenviron
   #define __targv     __wargv

#else

   #define  _tmain       main
   #define  _tWinMain    WinMain
   #define  _tenviron   _environ
   #define __targv     __argv

#endif

See also

Creating ANSI and UNICODE versions of Windows programs in c
Some CRT sources

Index