This class II ungrounded plug with two flat parallel prongs is pretty much standard in most of North and Central America. At first glance, the Japanese plug and socket seem to be identical to this standard. However, the Japanese plug has two identical flat prongs, whereas the US plug has one prong which is slightly larger. Therefore it is no problem to use Japanese plugs in the US, but the opposite does not work often. Furthermore, Japanese standard wire sizes and the resulting current ratings are different than those used on the American continent.
Type A and B plugs have two flat prongs with a hole near the tip. These holes aren't there without a reason. If you were to take apart a type A or B socket and look at the contact wipers that the prongs slide into, you would find that in some cases they have have bumps on them. These bumps fit into the holes so that the outlet can grip the plug’s prongs more firmly. This prevents the plug from slipping out of the socket due to the weight of the plug and cord. It also improves the contact between the plug and the outlet. Some sockets, however, don't have those bumps but just two spring-action blades that grip the sides of the plug pin, in which case the holes aren't necessary.
There are also some special outlets which allow you to lock the cord into the socket, by putting rods through the holes. This way, for example vending machines cannot be unplugged. Moreover, electrical devices can be "factory-sealed" by the manufacturer using a plastic tie or a small padlock through one or both of the plug prong holes. For example, a manufacturer might apply a plastic band through the hole and attach it to a tag that says: "You must do X or Y before plugging in this device". The user cannot plug in the device without removing the tag, so the user is sure to see the tag.
Countries Using Type A Electric Plug/Socket - American Samoa, Anguilla, Antigua, Aruba, Bahamas, Bangladesh, Barbados, Bermuda, Bolivia, Brazil, Cambodia, Canada, Cayman Islands, China (without holes in blades and slightly shorter blades), Colombia, Costa Rica, Cuba, Ecuador, El Salvador, Guam, Guatemala, Guyana, Haiti, Honduras, Jamaica, Japan, Laos, Lebanon, Liberia, Maldives, Mexico, Micronesia, Montserrat, Netherlands Antilles, Nicaragua, Niger, Okinawa, Panama, Peru, Philippines, Puerto Rico, St. Vincent, Saudi Arabia, Tahiti, Taiwan, Thailand, United States, Venezuela, Vietnam, Virgin Islands (U.S.& British), Yemen.