Server mirroring is an expensive but effective strategy for achieving fault tolerance. It's expensive because each server must be mirrored by an identical server whose only purpose is to be there in the event of a failure. A less expensive technique that is becoming more and more popular is clustering.

Also see the Server Types page in the Quick Reference section of Webopedia for a comparison of server types.

ServerWatch - Guide to Internet Servers and Platforms
Includes ratings, industry news, extensive reviews, and downloadable server software.

The Truth About Servers
What makes a server a server? Is there a difference between an e-mail server and a messaging server? Should a firewall be considered a server in certain circumstances? These questions and more are answered in an extensive tutorial that cuts through the marketing hype and explains the basics of key server types.