It's a little hard to explain, so I'll start with a snippet from the Dan Masters wiring diagram.
The green wires are switched and fused power from the ignition switch (hot whenever the ignition is on). The U/LG and R/LG wires are power to the wiper motor, the dash switch connects the 12v input (from the green wire) to either R/LG for low speed or U/LG for high speed. That (and the ground) is all it takes to make the wiper motor run.
The other wires are to implement the park function. When the dash switch is "Off", it connects the N/LG wire to the R/LG wire. When the motor is away from the "parked" position, the park switch inside the motor connects 12v (from the green wire) to the N/LG wire. So the motor will continue to run until it reaches the park position, and the park switch opens.
That's a bit of a simplification, but covers the essentials. However, don't try to leave the N/LG connected to the R/LG all the time. When the motor reaches the park position, it actually shorts N/LG to ground to make the motor stop faster.