The question has come up before and you will find a pretty good explanation in the posts in this thread: Serial ports
Since that thread, there has been some slight changes related to the RS232 port that terminates in the jack connector. In the newest firmware, that is also connected to the Arduino environment (as it was suggested in the thread above). See Re: Accessing the UART Port of Galileo for a bit more info on that.
If you don't need that, you can simply ignore that part and use the other two ports as describes in the thread.
I hope it helps!