Friction Pad or Shoe Replacement
This is just normal wear and tear. They will need replacing every 50 000 to 100 000km.
Most disk pads have a small piece of metal known as a wear tab on them. When the pad is almost worn out it touches the disc which causes a high pitched squeal designed to tell you the pads need changing. If this is ignored or there is no wear tab you will hear a grinding noise when the pad is gone. Damage to the metal parts happens very quickly once this happens and braking becomes very poor.
Part of your WOF is to check these where they are visible. Your car will fail a WOF if the pads or shoes are nearly worn out.
The Pad on the left is partly worn and the pad on the right has gone through to the metal backing. This Right had pad also damaged the brake disc badly.