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Check my thead in General Hardware regarding linux-clusters 🙂
Basically, a cluster is a collection (2+) of PC's (can be x86, Alpha, Macintosh etc.) which are all linked together, usually via Ethernet (because it's fast and cheap). These nodes (every system in a cluster is called a node) all run a copy of the same OS (usually Linux, because it's fast and cheap). Together these nodes function much like a parallel system, with multiple CPU's, and an (possibly) insane amount of HD space and memory (every node has its own RAM and HD).
It's actually much like a supercomputer (except for a few critical differences), but for a much lower price.