[Shotimes] O2 sensor question

[Steve Tatro]

Generic question regarding O2 sensors.  Sending to both lists since
the Gen3 is OBD-II.

Can someone explain what the different O2 sensors do?  As far as I can
tell on every OBD-II car there are O2 sensors both before and after
each catalytic convertor.  Is one there for emissions purposes only? 
I know the with Ohio's E-check emissions testing there is now a simple
OBD-II plug-in test, but can't figure out how all this works.

Also I'm guessing that if there is a fuel mileage problem that's O2
sensor driven that the culprit would likely be the sensor before the
cat.  Correct?

Anyone who knows please fill me in.  Inquiring minds want to know!

Thanks,

Steve Tatro
Red/Black '93 with 181k miles
Monroe, Ohio