Eighty zillion different spellings, but it comes down to: Yoghourt isn't a cheese, technically, as you aren't dealing with the casein here. What you're doing is changing the lactose (the main natural sugar in milk)into lactic acid, by a process of fermentation, triggered usually by some very specific bacteria. The level of acid is enough to get the yoghourty bite, but not enough to denature the protein and make it into cheese. You can take the next step yourself: adding lemon juice to yoghourt and draining it makes a nice sharp fresh cheese.

Sorry that was a lunchtime quickie, I've got a meeting I need to go to ...