The type of motors used within RickBot
There are two main types of direct current electric motor in the world...

Standard motors
The first type of motor is the standard sort of motor where you apply a voltage and it starts to turn. This type of motor only has two wires coming out of it.
These are dead easy to interface. Some transistors and/or relays to switch the power on and off, and cater to current reversal to handle 'backwards'.
Unfortunately these motors suffer because tolerances and rotational speeds (usually being dependent on the applied voltage) provide no guarantee that two motors will turn in the exact same way. The effect of this would be 'wandering'.
Robotic devices using standard motors come in two forms. Either one motor drives both back wheels, and turning is implemented by changing the angle(s) of the front wheel(s). Or one motor controls one rear wheel and sophisticated methods try to ensure that the device doesn't wander... (or otherwise the designer says "to hell with it" and allows for these intolerances in the design).

Stepper motors
The second type of motor is the sort where to apply a voltage to the motor and it may turn a fraction before 'freezing' in a fixed position. Usually has four or five wires coming out of the motor.
These are a lot more complicated to interface because they work by alternating power to specific windings within the motor in a specific sequence.
These motors provide a fine degree of control. Each 'pulse' steps the motor one time in the nominated direction. One step could be something like 7.5º, so it'd take 48 steps to turn full circle. As you have to explicitly step these motors, assuming the loading doesn't stall the rotor, you will always know exactly how much the motor has moved, and it will be possible to keep everything in perfect synchronisation for hours - of course, practically every printer available from humble yet noisy dot matrix models to freaky-smelling laser printers use stepper motors for precise placement of the paper and the print head (or laser beam).
A disadvantage of stepper motors is that high torque motors are a lot harder to come by than their standard counterparts. For example, if battery life was no object we could build a robot using two car starter motors. These motors can turn an engine, so dragging heavy stuff behind the 'bot shouldn't be too hard. Now, where would I easily obtain stepper motors larger than those used within photocopiers and the like?
Now for another advantage - applying a fixed current to a fixed part of the coil will 'lock' the motor. This feature could be used as a crude form of 'brake'.
And... another advantage. Because we move the motor in steps, our chosen stepping rate will directly affect the speed. To do this with a standard motor requires pulse width modulation. Not so different, but less precise.

A further complication of stepper motors
Many types of this motor exist, such as the delta configuration. We shall not concern ourselves with this, because it is like saying the Wankel engine exists ... it does, but how many people in school mechanics class have actually every seen one in real life, as opposed to a diagram in a book or a little model.
There are two main types of stepper motor and each have plus points and minus points.

The first type of stepper motor is the unipolar design, where the four 'poles' are two windings which are centre-tapped. The centre tap is usually connected to the +ve supply and we ground the required parts of the windings to energise them and set up magnetic currents. By switching which poles are magnetised, you can cause the motor to turn. Reverse the direction of the stepping, the motor turns the other way.
Here is a schematic representation of this type of motor:

The second type of stepper motor is the bipolar design, which is similar to the above with two windings, only without the centre tap. This means you can wind the motor core in a more efficient way. A bipolar motor built on the same frame as a unipolar motor will deliver 35%-45% higher torque. The big complication is that these motors require polarity adjustment. Let's say coil #1 requires +ve on the left and -ve on the right to turn clockwise a step. Well, to go counter clockwise we'd need to apply -ve to the left and +ve on the right. This is usually achieved by a design known as an 'H bridge' (actually, most cheat and use and IC!) to flip the polarity as required.
Here is a schematic diagram:

 

What type of motor do we use?
Within RickBot, we shall be using two unipolar motors with a 7.5º step angle. This is purely because it is what I happened to salvage from an old printer. In any case, the outputs from the Amélie board are identical: left motor direction, left motor step, right motor direction, right motor step. It is possible to drive either type of stepper motor from this.

As it stands, the application code uses Timer2 to generate interrupts after the desired time interval has passed, to control the motor stepping. You could try hooking the step signal through a power transistor to directly switch a standard motor if you'd prefer to use one of those.