A virtual machine server is a computer that runs multiple instances of virtual machines. This usually requires a bit more processing resources than a typical workstation. Each virtual machine provides either a VNC interface or some other interface to allow remote interaction.
In some ways, this is a generalization of a VNC server. All VNC sessions in a Linux box run Linux as the underlying OS. However, instances of virtual machines on a virtual machine server may run different operating systems. Furthermore, each virtual machine on a virtual machine server is isolated from the other ones in terms of memory space, disk space and other resources. This improves the reliability and security of each virtual machine environment.
Virtual machine servers are commonly deployed for web server. This allows one power computer serve as multiple less power machines. Since there is only one physical computer, maintenance becomes less expensive.