| The "ILS" is a system to allow aircraft to land on a runway without visual cues from the outside: it has been in use in varying forms ever since it first appeared during World War II. A basic ILS system consists of two directional radio or microwave transmitters (the localizer and glideslope) as well as two or three marker beacons.
The localizer transmits on 108-112 mHz, the same range used by VOR stations: it tells the pilot where the runway is on the ground and which way it points. The glideslope transmits on 329-335 MHz and tells the pilot what altitude they should be at. Both beams are shaped like cones about 10 nm long: the localizer is 35 degrees wide, and the glideslope is 1.8 degrees wide.
Marker beacons tell the pilot how close they are to the runway. The "outer marker" denotes the point where aircraft are supposed to line up and begin their approach: it often coincides with an NDB transmitter to help pilots find the beginning of the approach. The "middle marker" is 3,500 feet from the end of the runway, where aircraft are supposed to be at an altitude of 200 feet. Some approaches also use an "inner marker": if the pilot cannot see the runway by the inner marker, they may not land and must go around or seek an alternate airport.
The traditional way to fly an ILS is by "flying the needles." A typical ILS indicator consists of two needles, representing the glide slope and the localizer, which tell the pilot which way to fly to intercept the approach. For instance, this reading:
_________
/ \
| ----|-- |
| O| |
| | |
\_________/
...means that the aircraft is low and to the left of where it should be. By turning the aircraft in the direction of the needles' intersection, the pilot brings the aircraft to the proper approach path. When the aircraft is on course, the indicator looks like this:
_________
/ \
| | |
| ---O--- |
| | |
\_________/
(Instruments vary from aircraft to aircraft: this writeup is not for navigational use.)
ILS-equipped runways also have a complex array of lighting. White lights mark the centerline and edges of the runway, and are embedded in both sides of the runway for the first 2,500 feet to mark the touchdown zone. The last 2,500 feet of the runway use amber side lights and red centerline lights to warn the pilot that the end is near. There are also no less than twelve different lighting systems used to guide pilots to the threshold of each runway, ranging from simple lines of white strobes along the approach path to complex grids of red and white lights.
The Federal Aviation Administration certifies each ILS approach as Category I, II, or III, depending on its features and reliability. Category I systems can only be used when the end of the runway is visible from an altitude of 200 feet and a distance of 2,400 feet. For Category II, the limits are halved to 100 feet and 1,200 feet. Category III approaches have no minimum altitude, and a minimum distance of 700 feet. During periods of heavy rain or fog, Category III systems are often the only safe way to land.
For more specifics on ILS, you can read the FAA's specifications at http://www.netvista.net/~hpb/ils.html, or check out the guide at http://www.allstar.fiu.edu/aero/ILS.htm. | 
