The outer, middle and inner markers operate at 75 MHz. The glideslope, as above, indicates height. The glidepath is the correct height, decending, that will bring the aircraft onto the numbers at the end of the runway. The localiser shows the course to fly to the numbers.
The entire system is called ILS - instrument landing system. Localisers are found in the low end of the VHF navigation band - 108-110 MHz. Glideslope operates at about 330-335 MHz. The frequencies are set up in pairs and the systems are defined by the VHF localiser frequency. The loc transmitters also have a morse ID, the glideslope has no audio capabilities.
When the correct ILS is dialed up and the plane is approaching close to on course, the pilot will adjust his decent to keep the localiser and glideslope needles centered - they're crossed needles - and if he flies the approach keeping the needles centered, he will arrive on the numbers. This of course is how bad weather approaches were made for years, and largely still are. GPS is in the loop, but it hasn't taken over - yet.
Scroll down for a pic of the ILS receiver (also used for VORs.)
http://www.flightsimaviation.com/aviation_theory_25_ILS_Navigation_part_2_The_Glideslope.html
