ILS, simply put is 4 beams of energy. The localiser, this is generated from the metal structure at the far end of the runway. It is two beams of energy that overlap in the middle, so if the aircraft is receiving equal amounts of the localiser then it is on the centreline. two much of one or the other and it is left or right of the centreline.
The glideslope, the same as the localiser pretty much. You can see a vertical structure about one third from the start of the runway. It beams out two amounts of energy. Too much of the lower one and you are below the correct glidepath, too much of the upper one, obviously too high! Equal amounts mean you are on a perfect 3 degree approach path, not at all airfields, and you will hit the runway on the money.
Obviously this is in simple terms but I hope some others will add to this!
http://en.wikipedia.org/wiki/Instrument_landing_system
This has a good picture