When overheating, the temperature of the part rises above the operating temperature. Overheating can take place: 
*if heat is produced in more than expected amount (such as in cases of short-circuits, or applying more voltage than rated), or
*if heat dissipation is poor, so that normally produced waste heat does not drain away properly.
Overheating may be caused from any accidental fault of the circuit (such as short-circuit or spark-gap), or may be caused from a wrong design or manufacture (such as the lack of a proper heat dissipation system).
Due to accumulation of heat, the system reaches an equilibrium of heat accumulation vs. dissipation at a much higher temperature than expected.