What Wendy said is pretty much corrrect.
A 0-1volt narrowband 02 sensor is only accurate at stoich (14.7:1 afr). Outside of that point, it can tell lean or rich, but because it becomes pegged not far away in either direction, it is pretty much useless as a gauge.
A 0-5 volt wideband is generally accurate between the "wider" 10:1 and 20:1 afr window. It functions the same way, just with more resolution.
