OVF is a format that describes virtual appliances; these could be single or multiple machine. In doing so, OVF describes the virtual hardware and physical requirements of each machine.

Via Importing Kensho OVF to ESX | Citrix Blogs.

In my example I am going to use XenConvert to create an OVF Appliance from a XenServer XVA (that is an export of a XenServer VM), modify that using a VMware created OVF, and then import to ESX.