How is VMware ESXi different than VMware vSphere?
VMware ESXi is a hypervisor that partitions a physical server into multiple virtual machines. VMware vSphere is the industry’s first cloud operating system that drastically reduces ongoing costs and increases control over delivery of service levels while still preserving the flexibility to choose between any type of OS, application and hardware architecture. VMware vSphere offers organizations high availability and centralized management functionality that span across multiple ESXi hosts such as live migration, protection against hardware failures, power management, and automatic load balancing. VMware vSphere relies on a hypervisor to partition servers. Customers can choose to deploy either VMware ESX or VMware ESXi as part of the VMware vSphere suite. All the functionality of VMware vSphere is supported on both VMware ESX and VMware ESXi. In fact, VMware vSphere supports server resource pools that contain both hypervisors.