Solaris is an "operating enviroment" developed by Sun Microsystems. It was originally developed to run Sun SparcStation machines. Now it runs on many other hardware platforms too. The Operating System in Solaris is SunOS and also has window environment (OpenWindows or CDE).

Good answer by Bengali - just to add... Solaris is an Operating System based on Unix (similar to Linux). So if you have had experience of running other favours of Unix such as HP-UX, AIX, etc you will feel at home. It was designed with networking and scalabilty in mind. So it runs equally well on a 1 CPU box as a 128 CPU box. It is one of the most secure operating systems you can load on to a computer. Historically, Solaris has not had many problems with Virus, Trojans etc. Until recently, Solaris mainly only ran on computers manufactured by Sun (as well as small numbers on Fujitsu Primepower boxes, Tadpole laptops with the Sparc processor) and on PCs - but it never really did well in the PC market. I think that is about to change. The full production release of Solaris 10 will be free to download from www.sun.com at the end of January 2005. This is a major improvment over previous releases, running much faster, the ability to run Red Hat Linux applications, improved security, availabilty etc. It looks like Sun is trying to steal Linux's thunder as this release appears to be aimed at PCs as well as Suns traditional servers. Probably the best thing about Solaris is that it has a binary application guarantee - basically if the applciatino was written correctly, it will still run without requiring changes in future versions of Solaris. They havent broken that for at least 12 years - probably a lot longer. If you want to run office applications - (Open Office / Star Office) or email and you want an OS that virus's spyware etc hasnt been a problem in the past then its excellent but if you want to run games - forget it. I installed a beta release of solars 10 on my laptop and it works well. But I still use windows xp to play games.