> What is the difference between “a supercomputer” and “a bunch of racks of computers”?
In addition to the other responses, I like pointing people to this talk[1] by Jeff Hammond for a comprehensive answer to this question (you can skip to the 11:15 timestamp).
That talk is from 2009 though. Nowadays companies regularly run jobs on commercial data centers which can include thousands of GPU cores, Infiniband networking and other specialized equipment. One can make a pretty valid case that we are approaching the ability to make an ad-hoc supercomputer for yourself from the GCP console.