A display DC is a device context, created by Windows, that an application uses to paint and draw a window. Windows prepares each display DC for output to a window, setting the drawing objects, colors, and modes for the window instead of for the display device. When the application supplies the display DC through calls to GDI functions, GDI uses the information in the context to generate output in the given window without intruding on other windows or other parts of the screen.
Windows provides five kinds of display DC: common, class, parent, private, and window. The common, class, and private DCs permit drawing in the client area of a given window. The parent and window DCs permit drawing anywhere in the window. Although the parent DC also permits drawing in the parent window, it is not intended to be used in this way.
Windows supplies a common, class, parent, or private DC to a window based on the type of display DC specified in that window's class style. Windows supplies a window DC only when the application explicitly requests one ¾ for example, by calling the GetWindowDC or GetDCEx function. In all cases, an application can use the WindowFromDC function to determine which window a display DC currently represents.