No worries, we are all learning, you are right, Visa is to enter the country, and extending their stay or OWP is to maintain their stay legally.What Mapple007 said is actually true, my apologies for the incorrect information before! Still learning what it all means and whatnot.
So from what information I could figure out (they never make it easy, do they?) the visa allows entry into Canada, while the work permit allows working (and I guess staying) in Canada for the specified period of time. If you wanted to leave Canada and return before the PR is processed, then you will need the visa extension.