Series and parallel as applied to dc motors refer to

SERIES:

If you wire two or more motors in series then as one motor comes under more load and it's back emf decreases, the voltage drop across the motor decreases. This causes more voltage across the motor(s) with a lighter load and therefor more power goes to the unloaded motor(s). When one motor stalls, it is almost a short circuit and all the power goes to the other motor(s).

A simple test to demonstrate this is to wire two small dc motors in series with a wheel on each motor. As you slow one motor down by touching the wheel, the other motor speeds up. It is an electronic version of a differential.

With my 6 wheel chassis, if the motors were connected in series then when one wheel came off the ground or hit loose sand, it would get more power feed to it while the motors with traction would loose power. The vehicle would get bogged andthe wheel in the air or loose sand would spin furiously.

Parallel:

By having them in parallel, they all try to maintain the same speed. When one motor gets more load than the others, it draws more power to compensate without affecting the others.

A simple test is to run a motor off of a 7805 voltage regulator and load it up. The motor will try and maintain the same speed by drawing more current and will only fail to maintain speed if the regulator fails to maintain the voltage. (put a heatsink on the regulator before trying this one).