Typically these are taxed like regular income. There are a handful of sites in Canada that pay their interns using "fellowships", in which case this is not taxable. You'll need to look at the individual residency brochures or ask the DCT after you interview.
Probably best to assume these will be taxed, and then think of it as a bonus if it's not.
Edit: I'm assuming you're Canadian and doing internship in Canada. I am Canadian and know nothing about US taxes.
As far as I know, they are taxed regularly. However don't worry too much....your salary will still be below the poverty line so we all still should qualify for most of the low income subsidies out there
If you're a US citizen, you have to file in the US even if you are getting money from/living in another place (e.g. Canada). So if you're a US citizen, regardless of what the Canadians consider your internship stipend (if it's classified as salary, fellowship, whatever), you'll have to file/pay in the US (although likely not much, since internship salaries are low). Not sure how it works if you're Canadian, though. Canada and the US have a tax treaty so that you will not be double-taxed (e.g., if you end up owing taxes in both countries, you get credit for the taxes you paid to one in the other, assuming you don't make hundreds of thousands of dollars, but I imagine that's unlikely!).
I was mostly being cheeky when I said that but after your comments I looked it up and it seems like our stipends are generally 1.5x the poverty line (as a single person). Then again the stipend values seem to range across the country. But don't forget to factor in the "worth" of the CAD vs the USD. Probably a bad deal if you're a US citizen doing an internship in Canada then transferring your $$ over.