A quick google will find you lots of info on Vsphere (formerly known as ESX): http://www.vmware.com/files/pdf/products/vsphere/VMware-vSphere-Standard-DataSheet-DS-EN.pdf
vSphere is an Enterprise grade hypervisor that is installed on bare metal (i.e. on dedicated (and supported) hardware). vSphere has a HCL (Hardware Compatability List) to guide you through what is supported. If you want to build your own vSphere server then search for vSphere or ESX "Whitebox".
vSphere has a ton of enterprise grade features for fault tolerance, resource management, moving Virtual Machines between servers and storage and so on. There is plenty of in depth documentation here: http://www.vmware.com/support/pubs/vsphere-esxi-vcenter-server-pubs.html
When you download vSphere to install it you get 60 days of use with all of the Enterprise features enabled. You will need more than one vSphere server and some shared storage to take advantage of these features. During or at the end of the 60 days you can register a key (from VMWare) that will downgrade your server to the free version (with reduced funtionality).
I don't have time to actually type much more - so hopefully this helps.
Finally if you have a powerful enough machine with Workstation installed you can installed vSphere in a virtual machine (though performance will suffer if you do this).
The benefits of vSphere over Workstation are many - including perforance, scability, centralized management, resource management, virtual machine migration, vsphere clusters, fault tolerance ...
vSphere is a great product - once upon a time I bought Workstation and used it alot - but once I built a vSphere box to test things out on I never looked back.