i got this code ATOM MyRegisterClass(HINSTANCE hInstance) { //create the window class structure WNDCLASSEX wc; wc.cbSize = sizeof(WNDCLASSEX); //filling the struct with info wc.style = CS_HREDRAW | CS_VREDRAW; wc.lpfnWndProc = (WNDPROC)WinProc; wc.cbClsExtra = 0; wc.hInstance = hInstance; wc.hIcon = NULL; wc.hCursor = LoadCursor(NULL,IDC_ARROW); wc.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH); wc.lpszMenuName = NULL; wc.lpszClassName = APPTITLE; wc.hIconSm = NULL; return RegisterClassEx(&wc); } and i am just wondering what ATOM means and what kind of data type that is

Atoms.

Its a WORD typedef which in turn is unsigned short, a 16 bit variable (on 32 bit CPUs at least), I've seen it before, but I dont know exactly why they have it defined.