Answer:
A 'compund wound' dc generator has shunt winding and a series winding. The shunt winding has a high resistance, and draws only a small current from the armature, but as it has many turns of wire on it, it generates a strong magnetic field. The series winding has only a few turns of wire on it, and, even though it has a large current in it, its magnetic field is usually less than that of the shunt field. Because of this, the compound generator behaves in most ways like a shunt generator, and is controlled in the same way. As it is self-excited, the compound wound generator requires 'residual flux' to start the generating process.
The series winding, if connected to AID the shunt field, can boost the field flux as load comes onto the machine and correct for the normal 'droop' in output voltage as the load increases. If set up properly, this correction makes the generator self-regulating. Because the fields aid each-other, this connection is called 'cumulative compounded'. This type of machine is the workhorse of the larger generator market.
If the series field winding is connected so it's field OPPOSES the shunt field, then the output voltage of the generator collapses as the machine is loaded up. This connection is referred to as 'differential compounded', and is only used for arc welding generators - they have a high voltage before the arc is struck; a fairly low voltage during welding operations, and if the rod sticks to the work, they shut the generator down without damaging it.