Why is one prong wider than the other?
Plug Polarization
Nowadays, many two-prong devices have one prong wider than the other. This is so that the device could rely (not guaranteed!) on one specific wire being neutral, and the other hot. This is particularly advantageous in light fixtures, where the the shell should neutral (safety), or other devices which want to have an approximate ground reference (ie: some radios).
Most 2-prong extension cords have wide prongs too.
This requires that you wire your outlets and plugs the right way around. You want the wide prong to be neutral, and the narrow one hot. Most outlets have a darker metal for the hot screw, and lighter colored screw for the neutral. If not, you can usually figure out which is which by which prong the terminating screw connects to.