I learned a new fact today. In place of the phrase "is comprised of", one should generally use the phrase "is composed of". Thus, the following is grammatically incorrect:
The United States of America is comprised of 50 states.
When people use "is comprised of", they usually mean more plainly "comprises". This would be more grammatical and in line with the meaning of "comprise" — "includes":
The United States of America comprises 50 states.
There is actually a relatively simple rule to distinguish this: "The whole comprises the parts; the parts compose the whole."
However, the word "comprise" actually goes a step further. To use the word "comprise" emphasizes that the statement is all inclusive thus the following would be the most correct:
The United States of America comprises 50 states, a federal district, and multiple territories.
Hat tip to Andrew McMillen for the article which put me on to this rule.