If you don't quite know what a fax is, first read the definition on the previous page. And if you're shaky on what a modem is, it might be smart to read that definition too. Anyway, a fax modem is simply a modem designed for sending and (usually) receiving faxes via your computer. Using a fax modem, you can send a document to someone else's fax machine without having to print out the document and then stand there and make sure the pages don't jam in the fax machine. In fact, if the other person has a fax modem attached to their computer, they can receive the document directly into their computer and read it on their screen. Of course, they can print it to their own printer and then have a copy on real paper instead of that disgusting fax paper.
Apple's PowerBook (the laptop Macintosh) and many notebook pcs have fax modems built into them. You can take the PowerBook to your hotel, unplug the hotel room phone, plug that cord right into your PowerBook, and fax away. Incredible. In fact, if you need a printout and you don't have a portable printer, you can fax the document to yourself in the hotel lobby.
You can buy fax modems on add-in boards that plug into your computer, or in little boxes not much bigger than a deck of cards. Before you lay your money down, be sure you ask whether the fax modem can receive (some don't-they cost less, but you'll be stuck when someone wants to send you a fax). And ask how fast the fax modem is-the standard speed is 9600 bps, but some only run at 4800 bps. And find out for sure what software will work with it, because the modem won't do you any good if your software can't make it go.