ILS main component are:

Localizer

The localizer (LOC) ground antenna array is located on the extended centerline of the instrument runway of an airport,
remote enough from the opposite (approach) end of the runway to prevent it from being a collision hazard.
This unit radiates a field pattern, which develops a course down the centerline of the runway toward the middle markers (MMs) and outer markers (OMs),
and a similar course along the runway centerline in the opposite direction.
These are called the front and back courses, respectively.

The localizer provides course guidance, transmitted at 108.1 to 111.95 MHz (odd tenths only),
throughout the descent path to the runway threshold from a distance of 18 NM from the antenna to an altitude of 4,500 feet above the elevation of the antenna site.
The localizer course is very narrow, normally 5°. This results in high needle sensitivity.
With this course width, a full-scale deflection shows when the aircraft is 2.5° to either side of the centerline.
This sensitivity permits accurate orientation to the landing runway.
With no more than one-quarter scale deflection maintained, the aircraft will be aligned with the runway.

Glide Slope

Glide slope (GS) describes the systems that generate, receive, and indicate the ground facility radiation pattern.
The glide path is the straight, sloped line the aircraft should fly in its descent from where
the glide slope intersects the altitude used for approaching the FAF, to the runway touchdown zone.

The glide-slope equipment is housed in a building approximately 750 to 1,250 feet down the runway from the approach end of the runway,
and between 400 and 600 feet to one side of the centerline.
The course projected by the glide-slope equipment is essentially the same as would be generated by a localizer operating on its side.

The glide-slope projection angle is normally adjusted to 2.5° to 3.5° above horizontal,
so it intersects the MM at about 200 feet and the OM at about 1,400 feet above the runway elevation.
At locations where standard minimum obstruction clearance cannot be obtained with the normal maximum glide-slope angle,
the glide-slope equipment is displaced farther from the approach end of the runway if the length of the runway permits; or, the glideslope angle may be increased up to 4°.

Marker Beacons

All Markers operate on 75 MHz VHF (thus no frequency selections are necessary for the pilot) and radiate a fan shaped
field pattern giving to the pilot an indication of range from the threshold. The purpose of the markers is to provide range information while on the approach.

They transmit an almost vertical beam. Almost all installations are equipped with an outer marker and a middle marker.
Category 2 or 3 ILS may be equipped with an inner marker as well.
Audio- and visual signals in the cockpit will indicate when the aircraft is passing overhead.
In many installations, marker beacons are being replaced or supplemented by the use of a DME associated with the ILS.