Secure Socket Layers (SSL) is the standard way to implement secure web sites. By encrypting the traffic between the server and the client, which is what SSL does, that content is protected from a third party listening to the traffic going past.
The exact mechanism by which this encryption is accomplished is discussed extensively in the SSL specification, which you can read at http://wp.netscape.com/eng/ssl3/. For a more user-friendly discussion of SSL, we recommend looking through the mod_ssl manual, which you can find at http://www.modssl.org/docs/2.8/index.html. This document discusses not only the specific details of setting up mod_ssl, but also covers the general theory behind SSL it and has pictures illustrating the concepts.
In this chapter, we talk about some of the common things that you might want to do with your secure server, including how to install it.