In worldwide usage, a commonwealth, in essence, is an organized collection of countries that form under one government. A state is controlled by a particular nation that governs it through laws passed by the government.
In the United States In the United States, the distinction between a commonwealth and state is in name only while in other parts of the world, such as the British Commonwealth, the word "commonwealth" has a far different meaning. In the U.S., the terms "commonwealth" and "state" have basically the same meaning. Within the continental U.S., there are four states that are considered commonwealths: Kentucky, Massachusetts, Pennsylvania and Virginia. Different in name only, the laws in a commonwealth apply the same as those in a state.
As a side note, the succession of Virginia from the U.S. during the Civil War led to the formation of the state of West Virginia as it seceded from Virginia during that time span to remain with the Union. At the time, West Virginia decided to forgo the name commonwealth, thus becoming the state of West Virginia.
Where Commonwealth Originated While a commonwealth means different things depending on where someone is at in the world, the name commonwealth in the U.S. arose following the Revolutionary War. The name commonwealth is mainly a matter of distinction, with voters in the states referred to as commonwealths choosing the moniker when the state joined the U.S. as a state. Some attribute the use of the name commonwealth originally as a means to voice opposition to the monarchy of England, with commonwealth viewed as antimonarchical in nature.
Puerto Rico and the North Mariana Islands This can sometimes lead to confusion since commonwealths do exist within the U.S. that fall outside the definition above, namely Puerto Rico and the North Mariana Islands. Puerto Rico and the North Mariana Islands represent two commonwealths located outside the continental U.S. These two commonwealths differ in that while the citizens who live there have the same rights as U.S. citizens, they pay no federal taxes and have only nonvoting representatives in the U.S. Congress only.
Furthermore, residents of Puerto Rico have no votes in the Electoral College when voting for president and vice president, although they can participate in the presidential primary process that takes place before the general election. Residents of the North Mariana Islands who have lived on the U.S. mainland can cast a vote in the state where they last lived, meaning their vote counts toward the awarding of the Electoral College votes of their former states.
Other Uses of Commonwealth In addition to a commonwealth in the U.S., commonwealths exist in other areas around the world, although they tend to have a far different meaning. Commonwealth designates an association of sovereign states. This is best represented by British Empire and the various countries that it ruled and, in some cases, still does, such as Canada, India and Australia. Referred to as the British Commonwealth, the association contains more than one-third of the world's population and encompasses 52 countries. Other nations that are considered commonwealths include The Bahamas and Dominica.