Clusters are computers that work together to work for a common cause. There are two basic types of clusters. One is a HA (high availability cluster) and the second is an HP (high performance cluster). Ha cluster allows several machines to be a failsafe for one program such as a web server that can not have down time. The HP cluster allows may programs doing the same thing to act as one computer. Science or math intensive calculations are usually done on these machines. These are sometimes know as parallel systems. What we will look at here is the High performance cluster.
Special programming is usually required on the HP clusters, traditionally single computer applications will not work usually on these systems in parallel mode.
More info: http://idea.uab.es/mcreel/PelicanHPC/
See also: http://www.hpc.lsu.edu/help/pelicannewusersguide.php