The basic differences from a simple bunch of networked PCs are :
- The linux kernel is OpenMosix?. This not only allows transparent process migration between the nodes, but it also offers a cluster-wide common filesystem. Unfortunately, process migration is not always a nice thing to have (the overhead associated with continuously moving a job around the nodes makes migration a bad idea for long jobs, see below for the commands needed to disallow job migration).
- The availability of a proper (cluster-wide) queuing system (the Sun Grid Engine) supporting parallel (MPI-based) program execution. This is a nice system to have for large parallel jobs, but is probably too complex if you just want to run a simple stand-alone job.