LoadIcon function usage:

wndclass.hIcon = LoadIcon (NULL, IDI_APPLICATION) ;

sets an icon for all windows created based on this window class. The icon is a small bitmap picture that represents the program to the user. When the program is running, the icon appears in the Windows taskbar and at the left side of the program window's title bar. Later in this book, you'll learn how to create customized icons for your Windows programs. Right now, we'll take an easy approach and use a predefined icon.

To obtain a handle to a predefined icon, you call LoadIcon with the first argument set to NULL. When you're loading your own customized icons that are stored in your program's .EXE file on disk, this argument would be set to hInstance, the instance handle of the program. The second argument identifies the icon. For the predefined icons, this argument is an identifier beginning with the prefix IDI ("ID for an icon") defined in WINUSER.H. The IDI_APPLICATION icon is simply a little picture of a window. The LoadIcon function returns a handle to this icon. We don't really care about the actual value of the handle. It's simply used to set the value of the hIcon field. This field is defined in the WNDCLASS structure to be of type HICON, which stands for "handle to an icon."