A fire detector system is a set of sensors that monitor and detect a high termperature and warns the flight crew. Fire Detection Systems are usually installed in the engine under the cowlings. If it detects a fire, the flight crew can pull a lever that fires a charge that releases a chemical fire retardant and extinguish the fire.
There are other areas in an airplane that use Fire Detection System(FDS). FedEx cargo carrier is in the process of developing a FDS that monitors the freight containers (called "cans") that are positioned in the main flight deck. This system is a series of sensors mounted above each pair of Cans and watches for a hot spot. If it detects a Can that may have a fire inside of it, the computer commands the release of Argon from a central tank that operates an Arm. The Arm moves to the Can that is hot and jabs a point through the top of the Can. Then a special fire retardant foam is released under the force of the Argon gas and fills the Can, snuffing out the fire. This design will have STC approval in a couple of months.