Instrument Flight Rules (more commonly reffered to as simply "IFR") is a certain set of rules that a pilot must abide by when not flying with Visual Flight Rules (or VFR). During VFR flight, the pilot refers to things outside of the cockpit to determine position on a map - such as looking at the ground and identifying cities and mountains.

As one can imagine, keeping tabs on the ground can become difficult at night, on cloudy days, or in unfamiliar or sparsely settled territory. This is where IFR comes in. By planning a trip that uses various instruments in the aircraft, such as radio navigation equipment (VORs, DMEs, etc.), GPS systems, and the like, an aircraft can end up precisely where it wants to go without ever looking out the window.

The big problem with this is that if an IFR guy isn't looking out his window, he has quite a big chance at hitting someone else in mid-air; to counter this, IFR flights are required to be on radar at all times, constantly being controlled by ATC (Air Traffic Control) who then warns them about other conflicting aircraft.

As well, when flying IFR, you must fly at certain altitudes when heading in certain directions. This provides some inherent seperation and makes the Controller's job a bit easier.

Because IFR flight is a lot of math work and playing with electronics and talking on the radio, extensive training is required and an in-depth knowledge of how the Air Traffic Control system works definitely helps.

Because most equipment is not yet sensitive enough to place an aircraft on a runway without any visual help whatsoever, the final landing run is usually done using Visual Flight Rules; however, Automatic Landing Equipment and other such fancy things are coming down in price. It's just a matter of time before everyone has an airplane with nothing but an autopilot button!