Note: You are currently viewing documentation for Moodle 1.9. Up-to-date documentation for the latest stable version is available here: Moodle Clusters.
A Moodle cluster, in this sense, is a collection of computers that may or may not mirror a Moodle, but will load share. A lot of people could benefit if they understood how this could be done.
You might want to start here. Linux HA is a project dedicated to developing Linux Clusters.
This initially appears to be the same thing, but then reading it, The Linux Virtual Server project is very different.
Not everybody uses Linux though, For Windows Server 2003, the concept of Web Farming seems to be growing.
And from Microsoft itself, comes this article on Setting Load Balancing Parameters, also in Server 2003.
What is a "web farm"?
A web farm is a group of computers acting jointly as a single server and are located in a single location, that is, they share a common front-end dispatcher to distribute customer requests and share User demand, the load. Also known as a cluster, a web farm can be either a Web site that has more than one server, or an ISP that provides Web hosting services using multiple servers.
What is "load balancing"
Load balancing is dividing the amount of work that a computer has to do between two or more computers. All all users get served faster as the server gets more work done in the same amount of time. A combination of both hardware and software can be used to implement load balancing.