There are two ways to send faxes directly from a Mac.
One way is to use a USB phone modem such as the USRobotics USR5637. You set it up as a printer in the Print & Scan preference pane. One caveat: sharing the modem over a network doesn't work. Obviously, you need a phone line to do this.
The other way is to use an Internet fax service such as j2.com. You email the document to a server, and it's faxed to the recipient. A phone line isn't needed. This method is no more private than email, which is to say, not private at all.