A supercomputer is, generally, a really big and fast computer. The standard of what exactly constitutes a supercomputer has evolved with the capabilities of the computing industry. Supercomputers contain many processors and often take up entire rooms. They can do anything a normal computer can do, but usually much, much faster.
This category contains information about supercomputers, including (but not limited to) companies that manufacture supercomputers, supercomputing facilities, news articles about supercomputers, and supercomputing conferences.
Information about writing programs to run on supercomputers is in Computers/Parallel_Computing.
This category contains sites of companies that either sell supercomputers or sell time on supercomputers. While there are many types of computers that can be classified as supercomputers, this category is for the sale of the specific sort of supercomputer that is sold as one unit (if the product can be classified as a 'cluster', it's a parallel computer, not a supercomputer) and is not manufactured from commodity hardware (that's a Beowulf).