A guest operating system can be Windows, Linux, and other commonly used operating systems.
For the most recent list of guest operating systems that VMware products support, see the VMware Compatibility Guide site: http://www.vmware.com/resources/compatibility/search.php.
For instructions about how to install the most common guest operating systems, see the VMware Guest Operating System Installation Guide: http://partnerweb.vmware.com/GOSIG/home.html.