If you apply and get accepted to a program with co-op when you first start your studies in Canada, CIC should give you both a study permit and a co-op work permit. You need a co-op work permit to work during your co-op work term, just a study permit isn't enough. In some cases, like mine, co-op is an optional part of the program and you apply separately to your school's co-op program while you're already studying. Since I only have a study permit now, I had to apply for my co-op work permit separately.
I'm supposed to start my work term in January, but if CIC can't manage to process my application by then, I'll just have to sit at home and wait for the work permit to be granted before I can actually start working for my co-op employer.