Does the monorail at Disneyland and the metro in D.C. run on the idea of direct current motors? Since they reverse directions? Is it like plugging the train in backwards?

Those trains probably run on AC motors, because then they can use transformers to transfer power between circuits. Most likely, these trains use induction motors. To reverse the direction of the train, the engineer reverses the direction in which magnetic poles in the motors' stators circle the motors' rotors. When the poles reverse directions, the rotor has to reverse its direction, too, so that it chases those poles around in a circle.

What is the difference, if any, between appliances with a 2 prong plug and a 3 prong plug?

In the 2 prong system, current travels to the appliance through one prong and leaves through the other prong. The roles of the two prongs interchange every 120^th of a second. In the 3 prong system, there is one extra prong and that connects the frame of the appliance to the ground (the earth). This extra connection is a safety feature. If a wire comes loose inside the appliance and touches the frame, the frame can deliver charge and current to you through your hand and you can deliver it to the ground through your feet or your other hand. The earth is very large and a large amount of charge can flow into it without repelling further charge. Moreover most electrical systems are actually connected to the ground at some point. So if current can travel out of the circuit feeding power to the appliance and travel through you and into the ground, it will. You'll get a shock. The ground connection (the extra prong) makes sure that this extra current gets to the ground so easily that it skips you. So the extra wire prevents you from getting a shock if you touch a broken appliance. Plastic appliances often omit the extra prong because they have nothing dangerous to touch on their exteriors.