Short answer: Get your brakes checked if you hear squealing or grinding, feel vibration through the pedal, the car pulls to one side, the pedal feels soft, or a brake warning light comes on. Each points to a different part of the braking system.
Squealing
Many brake pads have a small metal wear indicator that squeals against the disc when the pad is getting thin. It is designed to get your attention. Some squeal also comes from glazed pads or missing anti-squeal shims.
Grinding
Metal grinding usually means the pad material is gone and the backing plate is touching the disc. Stop driving if you can: braking is reduced and the discs are being damaged with every stop.
Vibration through the pedal or steering
A pulsing pedal or shaking steering wheel under braking usually comes from uneven disc thickness. Discs are measured and replaced if they are below the minimum thickness stamped on them.
Pulling to one side
A seized caliper or slide pin, a collapsed brake hose or contaminated pads can make one side brake harder than the other. This also wears tyres unevenly.
Soft or sinking pedal
Air or moisture in the brake fluid, a leak, or a failing master cylinder can make the pedal feel spongy. Treat this as urgent.
Brake fluid
Brake fluid absorbs moisture from the air, which lowers its boiling point and corrodes parts over time. Many car makers specify a change every two years; your service booklet gives the interval for your car.
