Complex sentence: For major computer subsystems, a graphical file manager is a front end to the computer's file system, and a shell interfaces the operating system--the front end faces the user and the back end launches the programs of the operating system in response.
Split into simpler sentences: For major computer subsystems, a graphical file manager is a front end to the computer's file system, and a shell interfaces with the operating system. The front end faces the user, and the back end launches the programs of the operating system in response.