For beginners let me give a small introduction to clusters.
What are Clusters
A computer cluster is a group of linked computers, working together closely so that in many respects they form a single computer. Clusters are generally connected by a fast Local Area Network. Parallel programs that run on one of the nodes uses the processing power of all the nodes and produces the result. Generally clusters are tightly coupled ie. All the motherboards will be stacked into a single cabinet and connected using some interconnection network. They'll share RAM, Hard Disk and other peripherals. Operating System runs on one of the nodes and controls the activities of other nodes. For more on Clusters, refer the Wiki Page.