The following picture shows three valves in various states.  These valves were taken from a D series Honda engine.  The first valve on the left is normal.  A light buildup of deposits coats the valve however where the valve meets the seat it is relatively clean and smooth.  The middle valve shows a burned valve. The arrows point out marks created by hot gasses escaping past the valve during combustion.  A few things can cause this.  If the valve is slightly bent or out of shape a large area may be burned.  A few small areas such as this were probably caused by a dirty engine.  Deposits build up on the valve and valve seat and do not let the valve close properly.  The third valve on the right side shows mechanical damage.  A valve that is burned will start to pit and score; as the scoring worsens the valve will become weak and eventually break.