o2 sensor

i had this check engine light come on and had to get it checked cause as everyone knows it is annoying to have that light on. the computer analyzer said, rear upstream o2 sensor stays below 0.5v longer than 3 seconds. and there was another one that i can't remember right now. so i was told to get a rear o2 sensor before cat converter but websites only offer either o2 sensor before or after cat converter. is there any difference between rear sensor before cat converter and front sensor before cat converter? it makes me think that there are four, rear before, rear after, front before and front after. i don't think that's right but i don't know that much about cars, so i need help. what is the difference if there's any and can i get an o2 sensor before cat converter instead of the one i was told to get, rear o2 sensor before cat converter? thanks.

oh i have 95 dodge avenger es, 2.5 liter, 24 valve, v6.