differences between HW numbers and vmdk versions??

Where can I find something explaining the differences between hardware versions -- particularly HW version 8 and higher??

Does vSphere 5.1 and higher have a different vmdk version than 5.0??

I know about the hardware version compatibility idea introduced with 5.1 and beyond...but what about vmdk compatibility?? VMware Tools compatibility??

How much attention and concern must be paid to vmdk and VMware Tools differences between 5.0 U1/U2 and later ESXi versions??

Thank you, Tom

P.S. Surprised not to see anything yet in the blogosphere about vSphere.Next...prices should be reduced IMO now that Hyper-V has caught up in so many ways.