Why don't you tell us? There are better sources on the web for very basic information like how an RS-232 port works electrically. I would prefer that you do that research first, and then ask us questions if you don't understand what you are researching and reading. Please also provide web pointers to what you are reading and not understanding.
Google is your friend -- learn to use it well. Also, wikipedia.org is a fairly good source of information, as long as you remember that it is publicly-run and anybody can modify the information on it. For basic stuff like RS-232 DB-9 voltages, it's a good place to start:
http://en.wikipedia.org/wiki/RS-232
.