"__stdcall" is the Microsoft specific name of the "stdcall" calling convertion. Unlike __cdecl (the native calling convention of C/C++), it is supported in C/C++, Visual Basic, Java, and other languages alike, which makes it the first choice when building a DLL for cross-language use.
In Microsoft C++, identifiers with two leading underscores are reserved for compiler implementations. Therefore, the Microsoft convention is to precede Microsoft-specific keywords with double underscores.