ESX and ESXi refer generally to two versions of the VMware product line- these are the hypervisors that are part of vSphere. VMware refers to vSphere as a “virtualization platform,” which appears to mean an OS and the administration tools that ship with it. ESX/ESXi are the hypervisors – the critical piece of the operating system that allows multiple virtualized machine to run simultaneously. Several upgrade methods are provided.
ESXi is newer, more compact, and according to VMware, more secure. ESX was based on a stripped-down version Redhat, and loads a second OS kernel for virtual machines to talk to, which they call a vmkernel. ESXi consolidates this into a single process, removing on-board administration tools in favor of remote administration. While confusing, the distinction is largely irrelevant at this point – as of July, 2010 newer vSphere versions support only ESXi.
Newer versions of vSphere tout features running 32 virtualized CPUs, and distributing up to 1 TB of RAM to VMs, and support for up to 36 GB/s on the network. Many of their newer features appear to be device driver performance enhancements: dynamic resource allocation to VMs, compact device drivers, and support for large memory pages. The last is interesting- this is one of features that is supposed to make database virtualization tenable, as Oracle likes to grab lots of memory and manage it itself, rather than use general purpose OS paging features. It is also worth noting that only the newest versions of vSphere support Windows 8/Windows 2012.