In computing, a window ( aka form ) is an enclosed, rectangular area on a display screen. Most modern operating systems and applications have graphical user interfaces ( GUIs ) that let divide display into several windows. Within each window, may run a different program or display different data. Windows are particularly valuable in multitasking environments , which allow to execute several programs at once. By dividing display into windows, may seen the output from all the programs at the same time. To enter input into a program, you simply click on the desired window to make it the foreground process. GUI enable to set the dimensions and position of each window by moving the mouse and clicking appropriate buttons. Windows can be arranged so that they do not overlap (tiled windows) or so they do overlap (overlaid windows). Overlaid windows (also called cascading windows) resemble a stack of pieces of paper lying on top of one another; only the topmost window is displayed in full. You can move a window to the top of the stack by positioning the pointer in the portion of the window that is visible and clicking the mouse buttons. This is known as popping. You can expand a window to fill the entire screen by selecting the window’s zoom box. In addition to moving windows, changing their size, popping and zooming them, you can also replace an entire window with an icon (this is sometimes called minimizing). An icon is a small picture that represents the program running in the window. By converting a window into an icon, you can free up space on the display screen without erasing the window entirely. It is always possible to reconvert the icon into a window whenever you want.