To define a virtual cluster I have to explain what a cluster is. I will be very brief on explaining what a cluster is in order to explore more of the virtual clusters. A cluster is group of computers connected to each other in order to make them work as a single system. The most common way to form a cluster is to connect directly to an ethernet switch. Each computer is called a node, which represents each slave to the cluster. The master node accepts the user input and gives commands to the cluster. A cluster can be put together with group of cheaper computers which is much more cost effective, this increases performance and availability compared to a costly computer.
The picture above represents a Beowulf Cluster, the nodes connect to the ethernet switch waiting to receive orders from the Server (Master Node). There is also a Networked Disk Storage or Network attached Storage (NAS) attached to the network. This disk can be used for multiple things and can greatly affect how the cluster is managed. The storage can act as a file server and share itself with the nodes so it can be used for processed files, temporary storage or even for Diskless Nodes which is a more complicated form of a cluster.