The "black box" is made up of two separate pieces of equipment: the flight data recorder (FDR) and a cockpit voice recorder (CVR). They are compulsory on any commercial flight or corporate jet, and are usually kept in the tail of an aircraft, where they are more likely to survive a crash.
FDR records at least 88 required parameters about the flight, including airspeed, altitude, rudder position, wheel position, air pressure, and (if you believe the flight attendants) who still has their cell phone on at takeoff. CVR records everything the folks in the front cabin are talking about! While the old magnetic-tape versions could store up to 30 minutes of talk at a time, the current digital recorders can record up to two hours (which, honestly, still seems low). Once that two hours is reached, the CVR records over old material.
Essentially, a black box flight recorder is heavily protected recording device, similar to a hard disk or a memory card. The black box records all relevant flight data, in addition to conversations in the cockpit. Previously, this data had to be recorded on two different devices. But today there are also units that can do both. According to regulations, however, every airplane must have two of these devices on board. The voice recorder logs all sounds in the cockpit. In addition to discussions between the pilots, it also records automatic computer announcements, radio traffic, discussions with the crew and announcements to the passengers. The sounds of switches and engine are also recorded by the device.