Ron Lindenfeld offers us an extensive answer:
When used properly, the Flight Management System (FMS) increases the situational awareness, safety, and efficiency of any flight using this equipment.
The FMS is basically made up of the Flight Management Computer (FMC) and the Control Display Unit(s) (CDU). It's most basic function is to allow the crew to program a route from one destination to another, then engage it with the autopilot/flight director and allow it to fly the programmed route. The FMC has several databases that store waypoints and pre-programmed routes. The FMC is is accessed via the CDU. The CDU has a keypad and buttons that allow pilots to make entries into the FMC. A typical modern airliner/business jet has 2 or 3 cdu's - one for each pilot, the third for redundancy.
Without an FMS, pilots must keep tuning in the next VOR and fly the appropriate VOR radials in order to stay on the assigned jet routes. Doing this at the high speeds of high altitude flying increases heads down flying. A modern FMS will do all the radio tuning automatically, and track the programmed route for you. Most FMS's also includes SID's and STAR's in their databases.
These are basic functions. There are many more advanced functions that are available.
See the description in the FMS page...Back to the Flying page