A supercomputer is an extremely high speed computer. Usually a very powerful mainframe.
A supercomputer is an extremely high speed computer, usually made for things like simulating the inside of a sun, simulating the inside of an atomic bomb, molecular modelling, etc.
A brief way to describe is is an extremely powerful mainframe.
A list of the top 500 supercomputers can be found here.
POssibly other related tags are: cluster and paralel-processing