The Oxygen(O2) Sensor is installed in the exhaust system to determine the amount of oxygen left in the exhaust stream after combustion. This is used by the ECU to determine if more or less fuel (longer or shorter injector duration) is need to achieve optimum air/fuel ratios.
Earlier honda engines (OBD0 and prior) used a single wire oxygen sensor that sent a signal directly to the ECU. Later models (mostly OBD1 and newer) use a 4 or 5 wire oxygen sensor that include an integrated heater ciruit to assist in sensor responsiveness. These are labeled "Heated Oxygen Sensors" (HO2S)
NOTE: Most stock oxygen sensors are designed to only tell the ECU if the engine is running rich or lean, but not by how much. Anytime altered fuel ratios are desired, or the engine needs to be tuned for modifications, a wideband oxygen sensor is required. Click here for more info on Wideband Oxygen Sensors