An aircraft is said to have a glass cockpit if the primary flight instruments are digital and use flat panel or CRT computerized display screens rather than analog gauges. Although military forces were the early adopters of this technology, the plummeting costs of both computing hardware and of reliable and light flat panel displays means that civil air transport and general aviation aircraft are increasingly turning to this technology.
Although nearly all aircraft retain a minimal set of older independent instruments (colloquially known as "steam gauges") for use both to verify the glass cockpit systems and to provide backups in case of their failure, more and more aircraft are sold new or retrofitted with these systems. In general aviation use, a small glass cockpit airplane generally contains two display panels, known as the primary flight display (PFD) and the multifunction flight display (MFD). Some systems have these panels duplicated at each pilot station, or have two PFDs with a single MFD between them, etc. The PFD contains the same information as the'classic' instrument set - airspeed, heading, altitude, rate of climb, artificial horizon, turn coordinator. It may also display a computer-generated version of the terrain visible ahead of the aircraft, either from radar imaging or from a terrain database coupled with accurate GPS navigation and attitude instruments - in this case, it is called synthetic vision. It generally cannot be changed from displaying this information, so that the pilot always has a consistent layout and image to use. The MFD as its name implies can be switched between various display modes - for example, one mode might display engine and fuel information, while another might provide a navigation display and a third might control auxiliary aircraft systems.
Generally, one or more of the displays will have moded controls around its perimeter; usually buttons whose function can be changed in software for the appropriate context.
One problem with glass cockpits is that they are expensive; a Garmin G1000 system with a PFD and MFD (one of the 'standard' systems in the United States for small aircraft) can cost $50,000 (although that system is only installed new by manufacturers and is generally not available for retrofit)!
There is debate presently about the virtues of learning to fly in a glass cockpit. On one hand, they are excellent at managing the pilot's workload and information processing; they can allow more systems to be available to the pilot (weather display, navigation display, even nearby traffic displays, checklists, etc.). They can save weight, space and power as well as being easier to read. On the other hand, they are undeniably complex devices not only to read but to program and use, they can take too much of the pilot's attention during complex user interface tasks, and of course if the aircraft loses avionics power the pilot will lose all of their instrumentation rather than just a subset (this is why analog gauges are installed for backup).