Thunderbolt is what one may consider a physical layer,
that is hardware. What is carried on Thunderbolt would
be considered a transport layer.
The transport layer on Thunderbolt consists of two
protocols. Display Port protocol is one which supports
direct video interface. The other is a 2 channel PCI
Express protocol.
On the Mini (2011 or newer), if one is running Windows 7
via Bootcamp, the Thunderbolt Port appears as an extension
of the internal PCI Express bus. Any card adapter (of which there
are a couple available) will basically route these signals into
a card edge connector in the box any card plugged in will
appear in the Windows environment.
With OSX however, it has an additional layer of complexity
to the Thunderbolt interface by requiring that a driver be
"Thunderbolt Aware".
The one idea of the Thunderbolt standard is to make available
an external expasion bus that can be as flexible as possible
which was one of the reasons for the PCI Express protocol.