Abbreviated as UWB, this is a method of radio communication that basically takes the idea of spread spectrum communication to the extreme. It is a true digital radio communications system, which works by sending very very brief pulses (usually on the order of nanoseconds) of very low power across every frequency in the electromagnetic spectrum usable for radio transmission, usually the frequency band from almost DC to about 2 GHz. From the point of view of traditional radio equipment it looks just like low-level noise that one usually would filter out. To turn that noise into data that is received, the UWB receiver must know when to listen in order to detect the signal, so devices that would communicate this way need to be almost perfectly synchronized. A specific pattern of pulses is sent by the transmitter in order to make communication, and time shifting patterns in this pattern are interpreted by the receiver as data.
This makes UWB technology sound almost too good to be true. Because you need to achieve synchronizations of the order of picoseconds among your communicating devices to allow them to communicate, eavesdropping is about as difficult as cracking the best cryptography available. Because the actual signal is spread over many millions of frequencies over such a wide band, jamming is also equally impossible, as is interference. Since the frequency range used by UWB includes the extreme low frequency band that is used by submerged submarines to communicate, it can be used with no problem in places where most traditional narrowband radio communication is impossible, e.g. inside concrete buildings or underground. Since UWB timing schemes are essentially random in nature, the number of transmissions that can be multiplexed is essentially unlimited.
The data carrying capacity of UWB is enormous, given the spreading of so much data over so many frequencies. Prototypes made by the Time Domain Corporation and other researchers into this technology have produced results of 60 megabits per second or so, and theoretically gigabit rates are possible, as the technology improves.
The weakness of UWB is range, as it trades bandwidth for distance. With high-gain antennas, the maximum range at modest data rates is about 1 kilometer, while operating at the highest data rates range can be measured in only a few tens of meters.
Robert X. Cringely's Pulpit, January 24, 2002: http://www.pbs.org/cringely/pulpit/pulpit20020124.html
"Ultra wide band tunes up", ZDNet, http://www.zdnet.co.uk/itweek/brief/2001/33/network/
Time Domain Corporation website: http://www.timedomain.com/
FCC consideration papers for Ultra-wide band technology: http://www.fcc.gov/oet/waivers/