A guest operating system is an operating system installed inside a virtual machine. A guest operating system can be different from the host operating system (for example, you can run a Linux distribution in VMware Player installed on a Windows machine).
Many guest operating systems are supported by VMware Player, such as Windows 98, Windows XP, Windows 7, Windows 8, Windows Server 2008, Windows Server 2012, Solaris, Debian, CentOS, SUSE, FreeBSD, Red Hat, etc. To see the full list of the supported guest operating systems, go to this link. VMware Player is not listed, but the information for Workstation is applicable to Player.
Just like with the operating system installation on a physical computer, you need to have an installer disc or an ISO image file of the operating system you would like to install. An ISO file (also called an ISO image) is a single file that replicates the contents of an optical disc. ISO files are used to distribute large programs over the Internet, including operating systems such as Microsoft Windows or Linux. For example, here you can download an ISO version of Damn Small Linux and use it to install this Linux distribution in VMware Player:
In the picture above you can see that we are running Damn Small Linux (the guest operating system) inside VMware Player installed on Windows 7 (the host operating system).