The OVA is a pre-configured VM. It's created by F5 to make it easy for you to take it and drop it onto VMWare (It's what you'd call a software appliance).
However you also have the ability to just install from the ISO on a VM of your own choosing. Because at the end of the day, it's 'just' another Linux host running some rather clever software.
Once it's installed in a VM (With the appropriate compatible drivers etc) as far as the host is concerned, it's just a slightly different sized & specced BigIP.. (e.g. You can vary HDD size, numbers and types of virtual network interfaces, CPU's, memory etc).
H