You can read the official scribe here, but basically the neatest part about it is the way developers can create URL paths to an OVF file that we (users) can plug into VirtualCenter and import configured Virtual Machines or Appliances. This is very different from the past were we would have to download VMDKs, upload them to ESX hosts and then create the VMX files to properly set the CPU, Memory and disk locations.
The guys over at vKernel are the first I have seen use it and I LIKE IT! After popping in a URL pointing to their new SearchMyVM appliance (Read about it here), VC knew exactly how to configure the machine (in this case 4 hard drives,mem, cpu etc..) and began importing the machine.
For me, without OVF, this really would have been a pain in the neck to set up and I probably would have passed on the Search My VM appliance.