Different types of operating systems and versions available as web hosting server. As per your sites requisite and according to your knowledge you have to choose operating systems.
* Linux:- An opensource technology product. There are different flavours of linux like red-hat, cent os, suse, gentoo etc.
* Windows:- At present there are different versions of windows are available in market. Developed by microsoft corporation. Able to access windows GUI through remote desktop connector.
* SunSolaris:- Sunsolar is based on UNIX and developed by sun microsystems. Solaris 10 is a latest version of sunsolaris operating system.