Running ESX 4 as a VM within ESX allows administrators to provision a smaller quantity of capable hardware to do testing of the base product features. Of course, this isn’t what you would want to configure a production environment for (or purchase the related licensing, for that matter) – but simply a space to get down and dirty with the base functionality. It would also be a good idea to set up a NFS or iSCSI storage system to go through the drills with VMotion, Storage VMotion, Fault Tolerance, maintenance mode, host profiles, the vNetwork Distributed Switch or other features.

VMware employee Eric Gray gives the full details of this great training tool on his VCritical blog.