Current model vehicles utilize O2 sensors before and after each catalytic converter. The purpose of the front O2 sensor (before the catalytic converter) is to measure the exhaust gases exiting the combustion chambers to determine how rich (low oxygen content) or lean (high oxygen content) they are. Depending upon whether the exhaust gas is lean or rich, the ECU adjusts the amount of fuel going into the engine to try to maintain an ideal mixture that produces the lowest emissions output.
The purpose of the rear O2 sensor is to monitor the oxygen content of the exhaust gases leaving the catalytic converter. This provides the emissions system with a way to monitor the operation and efficiency of the catalytic converter.
Pictured is a front oxygen sensor