Less is a free, open-source file pager. A pager is a program that displays text files. Other pagers commonly in use are more and pg. Pagers are often used in command-line environments like the Unix shell and the MS-DOS command prompt to display files. Windowed environments like the Windows and Macintosh desktops don't need pagers as much, since they have other methods for viewing files. Less is not an editor. You can't change the contents of the file you're viewing. Less is not a windowing system. It doesn't have fancy scroll bars or other GUI (graphical user interface) elements. It was designed to work on simple text-only terminals.