VMware vSphere® uses virtualization to transform individual data centers into aggregated computing infrastructures that include CPU, storage, and networking resources. VMware vSphere manages these infrastructures as a unified operating environment and provides you with the tools to administer the data centers that participate in that environment.
The VMware vSphere stack comprises virtualization, management, and interface layers. The two core components of vSphere are ESXi and vCenter Server. ESXi is the virtualization platform where you create and run virtual machines and virtual appliances. vCenter Server is the service through which you manage multiple hosts connected in a network and pool host resources.
You can learn about vSphere by reading the documentation and watching videos on the VMware Information Experience video channel like this one.