Take a look at this VMware KB comparison between ESX and ESXi:
ESX(i) 4.1 - http://kb.vmware.com/kb/1023990
ESX(i) 4.0 - http://kb.vmware.com/kb/1015000
VMware vExpert 2009,2010
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Best explanation i have ever seen:
I use this place to thank David Davis.Thanks a lot.
I give you a direct answer of you direct question. vSphere ESX and ESXi are both bare-metal hypervisor architectures that install directly on the server hardware. The difference resides in the architecture components and the operational management of the vSphere host. Although neither hypervisor architectures relies on an OS for resource management, the vSphere ESX architecture relied on a Linux operating system, called the Console OS (COS) or service console, to perform two management functions: executing scripts and installing third party agents for hardware monitoring, backup or systems management. In the vSphere ESXi architecture, the service console has been removed, drastically reducing the hypervisor footprint (150 MB instant of 2 GB) and completing the ongoing trend of migrating management functionality from the local command line interface to remote management tools. The smaller code base of vSphere ESXi represents a smaller “attack surface” and less code to patch, improving reliability and security. The functionally of the service console is replaced by remote command line interfaces and adherence to system management standards.