Clustering is the concept whereby multiple servers can work together as a single virtual super-server. It is used to provide scalability (more work than a single machine alone can perform) as well as availability (one machine in the cluster can fail without causing the server to fail). Cluster originated with Digital Equipment Corporation (DEC) in the 1980s. It linked together multiple VAX VMS systems in such a way that multiple machines worked together as a single machine. Let's say that you were logged into one of the machines in the cluster. If that machine crashed, you could simply logged into another machine in the cluster and continue working.
These days, must clustering products deal with caching HTTP connections. This can be outgoing caches, or incoming reverse proxies designed to speed up websites.
