It sounds like the brake caliper has seized. If you know what you are doing, you can attempt to salvage the caliper set by taking it off the car and pulling the piston out of the housing and cleaning it up. But your best bet would be to just replace the whole brake caliper. Its not too hard a job if you know what you are doing and have a haynes manual around. But if you are not the type to turn a wrench, you need to bring your car in to be serviced ASAP. If you don't fix this soon the brake pads will wear down to the point that the whole pad drops out of the caliper and then the piston inside the brake caliper will be making contact with the brake discs. This isn't just bad for the car, its dangerous too.