Dude you can't be that young
SUN was a developer of UNIX based machines, using RISC processors, and thus a lot of early computer graphics software was written for this platform as well. Another early platform was DEC and their Alpha based systems.
Silicon Graphics came a little later to the game as far as I know, being specialized in graphics instead of general high-end computing.
Eventually most of these companies were beaten by PC based hardware becoming as powerful (with 64-bit support, and Linux) and cutting below their prices through economies of scale. I remember it was really big news when ILM has transitioned their artists from SGI systems to P4 based PCs...
SUn was bought by Oracle in 2010 or so. DEC was first bought by Compaq (?) and then HP bought Compaq a few years later...