From Peyton Hall Documentation
No, we're not talking about star clusters - not this time, anyway. We're talking about Beowulf clusters, or to use a more general term "computer clusters". Not the same as what OIT calls their "computer labs".
So what is a cluster? It's a set of machines, usually (but not necessarily) on a private network which is attached to a dual-homed "master node". Dual-homed means it sits on two networks at the same time, and may even act as a router between the two. This master node can allow logins, and is where you setup your large parallel jobs. Once the job is submitted, software on the master connects to the drones and runs the job there. This software is designed to fairly execute programs when there is available resources for them, and make sure that someone doesn't start a job on the same nodes that you're using for your processes so that everyone's programs get fair share of the machine.
We used to have a cluster of our own named "Hydra", however it was finally decommissioned in 2012 after having been converted to a general Condor cluster and slowly dismantled. The head node lives on (in spirit, at least) as the controller for our Condor infrastructure, which lets you submit jobs to run during idle cycles of desktops and other machines around the department.