Chrysler Repair: WHAT IS AN 02 SENSER,DONT..., free oxygen, crystel

Question
WHAT IS AN 02 SENSER,DON'T REALLY KNOW PLEASE HELP.  

Answer
the o2 senser tells the control system how rich or lean the
engine is running.It does this with a crystel in the sensor
that gives off different amounts of voltage depending on how
much free oxygen is in the exhaust at different locations.
The car makers still use the senser but the info it gathers
is not as valuable as it once was, now that the control systems are so much faster and better,however they can still
set a code in the engine computer,thuss they sell very well.
anything else? all sensors are now heated to close the loop
faster and pass strict imission standers in ca. tm