This is very fundamental stuff covered my many documents in the printed media and the web. In Rs485 most devices are master/slave - 1 master, many slaves. The master sends a packet of data which has an address for the slave. The slave in turn decodes the packet, decides if it is it's address, and responds if it is.

A typical protocol used in industry a lot is MODBUS .see www.modbus.com there is also a 9 bit protocol whereby each character sent has 9 bits, if the 9th bit is set, that character is an address otherwise it is data. see spec sheet on the 8051 for a description.

You can also decide to cope with collisions on RS485 - a simple way is to measure the current of the 485 driver or you can look at the characters you are sending - if they get corrupted you have a collision or bus problem. you can then decide to stop sending and retry later much like ethernet. this method get a little more complex.