PHiZ is right but the main difference between the two is:
lets just say that there is a faulty o2 sensor in two different cars, one OBD1 and one OBD2, the problem with the o2 sensor will only have to occur once (OBD1) for the check engine light to come on, on the other car, the o2 sensor problem will have to occur twice(OBD2) in order for the check engine light to come on