I am not an expert at this but here it goes....
Over the Air (OTA) is "free". Just like channels were tuned in before cable. You set up the antenna and point it and hopefully you are able to pick up a signal.
How many channels are broadcast depends on what area you live in. Most major cities here in the USA broadcast a digital channel as well. You need a tuner to decode the signal which is different from the analog signal. That is what the ATI product does. It tunes in the analog and digital (HDTV) channels if you can pick them up with an antenna (large or small).
People get HDTV in the states from OTA, Cable, or Satellite. I get all mine from cable because my TV doesn't have an HD tuner built in. However I would be able to pull in most of my local channels digital broadcast (NBC, ABC, CBS, etc.)
I am guessing that you will NOT be able to tune in any HD channels with this ATI card with signals that are from cable or satellite. Thats why I think they explicitly state this is only an OTA card. As a side note this card has a software based decoder. With an hour of HD averaging about 10gig I hope you have a lot of cpu power on hand.
Based upon what you say Jason, I don't think this ATI card will do you any good if you can't get any HD channels (or analog channels) via antenna from where you live.
There is something called QAM that would allow you tune in (some) HD channels from your cable co. But I am pretty sure this ATI card does not have QAM support. BUT I don't think all cable cos use QAM and not all HD cards that support it seem to work well. That is about as technical as I can get on QAM.
This HD decoder
http://www.usbhdtv.com/ I think does have QAM support, plus it's a hardware based decoder, plus it's USB2 external. And they ship up to Canada. Right now this is the one I would like to buy.
There is also a DVICO
http://www.dvico.com/index.html that supports QAM. There are probably a few others out there as well.
Hope that helps out a little....