The radar is a map available to the player. It displays known parts of the battlefield and helps the player know what is going on. In most games its availability is tied to a building or unit the players controls. For lore articles see the Tiberium universe one, Red Alert universe one, and Generals universe one.
In later games (Generals, Tiberium Wars, Red Alert 3, and Tiberian Twilight) the player can always see the terrain. However, due to fog of war, enemy buildings and units would only be visible if the player had a unit or other means nearby to see it. Buildings in the fog of war will be seen in its last state until the player has sight of it again.
If the Nod Stealth Tank is cloaked it is also hidden from radar.
Both the Allies and Soviets employed the Radar Dome to provide radar.
If power is low the radar will be unavailable.
In Renegade the radar is always available.
The Allies have their radar provided by Airforce Command Headquarters and also by the Spy Satellite Uplink which reveals the entire battlefield. If the Spy Satellite Uplink is lost the shroud is reset.
If the power is low the radar is unavailable.
The USA gets their radar from their Command Center which also allowed periodic use of Spy Satellite ability to reveal parts of the battlefield. The USA also had the Intelligence ability provided by the Detention Camp in Generals or the Strategy Center in Zero Hour to reveal all enemies on the map. Lastly the Spy Drone Generals promotion could be deployed to reveal parts of the battlefield until the drone was destroyed.
China gets its radar from their Command Center with a Radar upgrade. In Zero Hour Satellite Hack 1 reveals the area around enemy command centers and Satellite Hack 2 reveals all enemies every 25 seconds. Both Satellite Hack upgrades are available from the Internet Center.
For the USA and China the radar would be lost if the power was low, a weakness the GLA do not have. Any stealth units and buildings would be hidden from radar unless detected.
Radar is disabled if power is low. All stealth and cloaked units and buildings are hidden from radar unless detected.
The Allied, Soviet and Imperial Construction Yards all provide radar if they are deployed. The Allies also employed the Surveillance Sweep Top Secret Protocol to periodically reveal parts of the battlefield. The radar is unavailable if the power is low and submerged sea units are not visible on the radar unless detected.
Radar is always normally available. GDI Support Class employs the Recon Drone Support Power to reveal part of the battlefield until the drone is destroyed.
The only counter to it is Nod's Support Class's Hack Radar Support Power which periodically disables enemy radar and shared viability.
Cloaked units are not visible on radar unless detected.