What is a virtual server?
A virtual server is similar to a dedicated server in that you have access to a "dedicated" machine, however that machine is one of several virtual machines running on the same actual server hardware so memory and CPU use is normally distributed between virtual machines. This has a cost advantage and virtual servers as normally cheaper that dedicated servers.
You can usually choose between different operating systems and memory/cpu specifications to tailer you server as you need it but excercise caution should your performance requirements of your application change suddenly when using a virtual environment.