A hypervisor is a virtualization software tool allowing distributing physical server resources among multiple virtual machines.
Using a hypervisor, you can create copies of virtual machines on the same physical server, each of which will look like separate hardware to the user. Memory and performance resources can be specified in the settings for each virtual machine. Hypervisors allow you to create isolated operating systems and even entire environments on a single server.
The largest hypervisor vendor on the global market is VmWare.