What's shakin' in Beantown?
It could be a couple of things, your brake shoes could be wore down where metal on metal is squealing when you apply them, bad news.
brake dust has acculmilated in the drum, good news.
this can be fixed by spraying brake cleaner (buy at auto parts store) through the small hole in the brake drum, spin the tire, apply brakes and spray some more.
(lay some newspapers down cause it will get messy)
the small hole I refer to should have a rubber plug in it, pop it out with screwdriver.