From Wikipedia:
A variation on the zirconia sensor, called the 'wideband' sensor, was introduced by Robert Bosch in 1994 but is (as of 2006) used in only a few vehicles. It is based on a planar zirconia element, but also incorporates an electrochemical gas pump. An electronic circuit containing a feedback loop controls the gas pump current to keep the output of the electrochemical cell constant, so that the pump current directly indicates the oxygen content of the exhaust gas. This sensor eliminates the lean-rich cycling inherent in narrow-band sensors, allowing the control unit to adjust the fuel delivery and ignition timing of the engine much more rapidly. In the automotive industry this sensor is also called a UEGO (for Universal Exhaust Gas Oxygen) sensor. UEGO sensors are also commonly used in aftermarket dyno tuning and high-performance driver air-fuel display equipment. The wideband zirconia sensor is used in stratified fuel injection systems, and can now also be used in diesel engines to satisfy the forthcoming EURO and ULEV emission limits.
My understanding is that the wideband is necessary if you have done extensive modifications (e.g., forced induction) that require the sensor to operate outside of its normal range. Jeffy has commented on the wideband O2 sensors in some past posts, you might want to search.
