RS-485 is like RS-232, a serial communication bus, but where RS-232 is limited by speed and length, RS-485 is transmitted as a balanced signal and can be transmitted over longer distances with higher speed.
RS-485 needs 4 wires and can also transmit to multiple receivers and is capable of speeds up to 10 Mbit and be transmitted up to 1200 meters, but not at the same time. The same rule goes as for RS-232. The longer the cable, the slower the transmission speed.
RS-485 is not a very commonly used bus in the AV business, but it is seen in some speakers, amplifiers, mixers and in some cases manufacturers modify the standard and labels it something else, e.g. Fohhn-Net. Another use of the bus is actually from the lighting world, where DMX512 is a derivation of RS-485.