Load balancing is used by large websites to spread out a computing load between multiple servers. A site might be load balanced between only two servers, or perhaps even several hundred! While these servers are expensive, the cost of many low-power servers is much less than a large mega-server.
The picture above (click for full resolution) shows how the Wikimedia Project’s servers are configured. As you can see, it uses more than 100 servers! Each one is specialized for a certain use. The dark green boxes represent Apache web servers and the light green cache servers. The blue are load balancers– servers that tell the other systems what tasks to perform.
Most load balancing setups aren’t this complex, but if you have a large website that is quickly outgrowing a very powerful server, the technology might be worth looking into.
Photo | Wikimedia