What is 'black box' in Flights ?

After the disappearance of Malaysia Airlines Flight MH370 shortly after midnight on Saturday, investigators considering a range of possible causes - mechanical failure or pilot error - have yet to turn up solid clues. They don't even know where the plane disappeared, which means that for grieving families this misfortune will remain a mystery until the recovery of the plane's 'black box'.
What is this 'black box'? It is actually a flight data recorder (FDR) or accident data recorder (ADR) is an electronic device employed to record any instructions sent to any electronic systems on an aircraft. It is a device used to record specific aircraft performance parameters. Another kind of flight recorder is the cockpit voice recorder (CVR), which records conversation in the cockpit, radio communications between the cockpit crew and others (including conversation with air traffic control personnel), as well as ambient sounds. In this both functions have been combined into a single unit. The term "black box" is popularly referred by the media. The data recorded by the FDR is used for accident investigation, as well as for analyzing air safety issues, material degradation and engine performance.
FDRs are usually located in the rear of the aircraft, typically in the tail. In this position, the entire front of the aircraft is expected to act as a "crush zone" to reduce the shock that reaches the recorder. Also, modern FDRs are typically double wrapped, in strong corrosion-resistant stainless steel or titanium, with high-temperature insulation inside. They are usually bright orange. They are designed to emit an underwater locator beacon for up to 30 days and can operate immersed to a depth of up to 6,000 meters (20,000 ft).