Declan, I can point you to a couple of online resources:
The Intel FPGA YouTube channel includes a short introduction of HPC at the link here. You can check out the first 4 minutes or so if you're interested in a quick, high-level overview.
IBM and InsideHPC published "Clusters FOR DUMMIES" a few years ago. It's available as a free download on their website here. It's a short book but it has some good info in there on clusters and why they're essential for HPC workloads. And, no, I'm not calling you a dummy, that's just the name of the book series :)