Runway Lighting is compiled of several different types of lights, Runway End Identifier Lights (REIL), Runway Edge Lights, and In-Runway Lighting.
Runway End Identifier Lights (REIL)
Runway End Identifier lights, or REILs, are a pair of synchronized flashing white lights placed on either side of a particular runway's threshold. Oftentimes these lights are in place only at airports where the runway is surrounded by other lights (i.e. in a city) or hard to distinguish from surrounding terrain. They may also be used in times of poor visibility to make runway recognition easier from the air. The main purpose of these lights are to aid a pilot in quickly and correctly identifying the runway threshold from the air.
Runway Edge Lights
As the name indicates, runway edge lights span the length of the runway and indicate the boundary of useable pavement. In addition, on instrument approach equipped runways, the color of the lights also indicates to the pilot the general remaining distance of the runway.
Generally, runway edge lights are white, but, as aforementioned, on instrument approach equipped runways the lights are yellow along the last 2,000' of runway (or, in the case of a runway shorter than 4,000' , the lights along the second half of the runway will be yellow). Additionally, the edge lights placed along the beginning of the runway will appear green to pilots on the approach side of the threshold, and edge lights along the departure end of the runway will appear red to indicate the end of the usable runway.
Edge lights are further classified by intensity, as High Intensity Runway Lights (HIRL), Medium Intensity Runway Lights (MIRL) and Low Intensity Runway Lights (LIRL). Hight and medium intensity lights tend to have controllable intensity to allow for dimming or brightening the lights per pilot's preference, while Low Intensity Lights have only the low intensity setting.
At some larger airports, generally those equipped with precision instrument landing systems allowing aircraft to land under conditions of very poor visibility, runways will be equipped with lights set into the pavement of the runway itself that indicate the runway centerline as well as taxiways entering and exiting the runway, runway touchdown zones, and other even land and hold short orders.
Runway centerline lights, similarly to edge lights, indicate through color the remaining distance of the runway. These lights are spaced at 50' intervals along the runway centerline and are white until the last 3,000' of the runway. At this point they begin to alternate white and red until the last 1,000' of runway at which point they become only red lights.
Taxiway lead on and lead off lights alternate red and yellow and indicate to aircraft and ground vehicles when they are in the runway safety are or ILS critical area. Once past these lights on the taxiway side, you are clear of the critical approach areas.
Touchdown Zone Lights (TDZL) are specific to precision approach equipped runways and consist of two rows of white lights spaced evenly on either side of the runway centerline, indicating the approved touchdown zone of the runway. These lights begin 100' after the runway threshold and continue to 3,000' (or half the runway distance, whichever is less) along the runway.
Land and Hold Short (LAHSO) lights are white and installed perpendicularly across the runway to indicate the hold short point. These lights will only be on when LAHSO is in effect.