Maximum PC

Looking at the HTPC build done a couple issues back and there was a big deal about the cablecard used.

What's the deal? Like what is it's function as opposed to a regular TV Tuner?

And is there an equivalent hardware in Canada? (I'm assuming the card is specific to the carrier you subscribe to or has to be enabled by them or something?)

Regular tuner: can tune what a TV can tune.
CableCARD tuner: can tune what a cable box (provider-supplied) can tune. That's a lot more channels. And, in their native HD, too.

I bought an ATI Digital Cable Wonder for my dekstop just over a year ago, and haven't had any regrets. That pushed me to get the InfiniTV4 for my main Media Center - again, no regrets. And, I've ditched the cable box completely - there's not a one in the house.
Good: watching Sanctuary in HD.
Better: watching Sanctuary in HD, using my Media Center (and its better UI, and its better Guide) to tune it.

As to CableCARD in Canada: it does happen; however, it's on a market/provider decision basis. If you're lucky, your local cable office can make a CC available to you. If you're not (most markets, from what I understand) so lucky, well, no CC for you. And, unfortunately, your CRTC hasn't mandated (because, as I understand, they can't mandate it) making CableCARD available.
Here in the U.S., all cablecos are required (by FCC mandate) to make them available.