RS232 uses non-balanced signals. RS485 has balanced signals which increases noise immunity radically.
You are right that several devices can use same RS485 wires. Of course only one of them can can transmit at a time, but all can transmit in turns. RS232 doesn't have that capability, each wire has one dedicated transmitter. RS485 needs some complex software to retain law and order if there's several possible transmitters on the same bus. That's a general networking problem and it has several common solutions.
RS232 defines how transferred bits and characters are presented (=timing) and handshaking for flow control, RS485 defines only bit voltage levels, one can use as complex timings as he wants.
ERRATA: Comments claim that the common asynchronous communication data format with start- and stopbits isn't a part of RS232 spec. User Sam Gibson has checked it from the current standard, so it's a fact and I was wrong.