What is difference between Load angle and Power angle

Load angle (or Torque angle): For a synchronous generator, the magnetic field rotates at synchronous speed and the rotating magnetic field is created in the stator. These two fields are not fully aligned. The stator field lags the rotating field. This lagging expressed in angle is called load angle.
Power angle: For a generator, the power angle is the difference between the generator induced voltage and the generator terminal voltage. The value of the power angle is same as the load angle. So, in context of generator, power angle and load angle mean same thing.
For the case of transmission line the power angle is the angle between the angles of the voltages at two different points (bus). The transfer of power between the two points of power system is proportional to the sine of this angle.

Power factor is the cosine of the angle between the current and voltage. 

Power angle is the angle between a generator’s internal voltage and its terminal voltage, or between the voltages at the source and load points of an electrical transmission line.

Power angle is also same in context of synchronous machines but it is more generic – also used in power transmission line. To explain what they are: Load angle (or Torque angle): For a synchronous generator, the magnetic field rotates at synchronous speed and the rotating magnetic field is created in the stator. But the fact is that Load angle or power angle is the angle between terminal voltage and excitation voltage. Torque angle is the angle between excitation field mmf and resultant air gap mmf….in a synchronous generator. The load angle and torque angles differ due to the presence of armature leakage emf.