A computer cluster consists of a set of loosely or tightly connected computers that work together so that, in many respects, they can be viewed as a single system. The source of this information is http://en.wikipedia.org/wiki/Computer_cluster.
There are multiple reasons why we do clustering of systems. Enterprises have requirements to grow which have to be matched with cost effectiveness and future roadmap of solutions; therefore, it always makes sense to go for clustered solution. One big machine to handle all the traffic is always desirable but the problem with vertical scalability is the ceiling in compute capability of the chip. Moreover, bigger machines always cost more as compared to a group of smaller machines with aggregate same compute power. Along with cost effectiveness, the other nonfunctional requirements that a cluster can take care of are performance, availability, and scalability. However, having a cluster also increases efforts of manageability, maintainability...
