VMware ESXi (formerly ESX) is an enterprise-class, type-1 hypervisor developed by VMware for deploying and serving virtual computers. As a type-1 hypervisor, ESXi is not a software application that one installs in an operating system; instead, it includes and integrates vital OS components, such as a kernel.
After version 4.1, VMware renamed ESX to ESXi. ESXi replaces Service Console (a rudimentary operating system) with a more closely integrated OS.
VMware ESXi is a smaller footprint version of ESX that does not include the ESX Service Console. It is available without the need to purchase a vCenter license as a free download from VMware with some features disabled.
VMware ESXi was originally a compact version of VMware ESX that allowed for a smaller 32 MB disk footprint on the Host. With a simple configuration console for mostly network configuration and remote based VMware Infrastructure Client Interface, this allows for more resources to be dedicated to the Guest environments.
There are two variations of ESXi, VMware ESXi Installable and VMware ESXi Embedded Edition. The same installation media will install to either one or the other of these installation modes depending on the size of the target media. It has the ability to upgrade to VMware Infrastructure 3 or VMware vSphere 4.0 ESXi.