@ robbie21- God bless you! I don't think I could be a teacher for anything.
I think the main difference is that a shop foreman's duties include actually getting his hands dirty (blue collar) while the manager does only administrative tasks (white collar). My father is a mechanic, and that's the impression I've always gotten from him. So if you're talking about a car repair shop, the foreman would be the head mechanic, while the manager might never have spun a wrench in his/her life. (That's changed. Years ago, you didn't have to have a college degree to be a service manager.)
Another example is actually from the movie Gone With the Wind. The overseer was a white guy (obviously of lower class than the planter), while the foreman was one of the slaves.