VMware ESX is an enterprise-level computer virtualization product offered by VMware, Inc. ESX is a component of VMware's larger offering, VMware Infrastructure, and adds management and reliability services to the core server product. VMware is replacing the original ESX with ESXi.
VMware states that the ESX product runs on bare metal. In contrast to other VMware products, it does not run atop a third-party operating system, but instead includes its own kernel. Up through the current ESX version 5.0, a Linux kernel is started first, and is used to load a variety of specialized virtualization components, including VMware's vmkernel component. This previously-booted Linux kernel then becomes the first running virtual machine and is called the service console. Thus, at normal run-time, the vmkernel is running on the bare computer and the Linux-based service console runs as the first virtual machine.