RS485 in and out

I'm new to the Arduino and I'm trying to get my bearings. I've purchased a couple Duemilanoves and I'd like to send RS485 in, use the Arduino to interpret the signal, and send either 485 or RS232 out. I've read that the Arduino can not take 485 in natively and have seen some info about using a MAX485 to alter the signal. What I can't understand is what the MAX485 is putting out that the Arduino can interpret. Is it converting to RS232, or just altering the bias of the 485? What is the native communication protocall the Arduino supports, or does it depend on what libraries or code you install on it? How many serial ins or outs can the Duemilanove support? Sorry if this is basic info and covered somewhere else, but I haven't been able to find it.

what the MAX485 is putting out that the Arduino can interpret.

The chip converts the voltage levels of RS485 into TTL voltage levels. Normally these voltage levels carry serial data at some baud rate.

There is normally only one serial port on an Arduino (one input and one output) although there are four on a MEGA board. You can use other pins with the software serial library.

What is the native communication protocall the Arduino supports,

Asynchronous serial communication.

but I haven't been able to find it.

You are having a ;D