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.
Workstation Player is not listed, but the information for Workstation Pro is applicable to Workstation Player. Operating systems that are not listed are not supported for use in a virtual machine.
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.