Brake squeal is caused by the brake pads vibrating when light to moderate brake pressure is applied. Heavy braking usually stops squeal because the extra pressure dampens the vibration. If heavy braking doesn't stop the squealing, it's a sign of a significant problem. Squealing under light braking isn't a safety concern, it's just annoying.
Most auto parts stores sell a compound that smears on the backside of the pad. Corrects squeal problems. I don't suggest the spray ones cause I've heard they don't work very well.
So the following info I found on a website that a bit more detailed.
Remove the pads. If they are glazed over (from braking too hard before they were seated), wet sand the surfaces with brake cleaner (so you don't generate dust). Take a file and bevel the leading edge of the pad about an eighth of an inch. (This prevents the leading edge from digging in quite so hard, so that the trailing edge won't be driven away so hard by pad rotation.
I usually don't find it necessary to supplement the anti-squeal springs. If the brakes still squeal, NAPA sells anti-squeal shims with an adhesive backing that stick to the back side of the brake pads. These work much better than the spray or brush on anti-squeal coatings that I've tried. I've usually only needed to use these when running combination track/street pads.
If pads develop squeal after a long period of use:
Try braking hard a few times. This might be successful in breaking through any glazing that's occurred or clearing away dirt.
If that doesn't work: Remove the pads. If the trailing edge of the pad is thinner than the leading edge, replace the pads (something else is probably sticking). Check to make sure the caliper is sliding freely on the pins. Clean the rotor. Wet sand the pads and bevel the leading edge.