An embargo is nothing more than a stronger sanction. An obstruction of the market. It is a form of prohibition, partial or complete, of commerce and trade with a country, state or a group of countries.
They are considered to be a strong diplomatic measure. Also, they are the most effective non-violent way to put out a message of indignation, noncompliance, disagreement or even of punishment to one's actions (in this case, to another country's policies).
Embargoes are usually considered a legal barrier to trades, not to be confused with blockades, that are usually seen as acts of war.
So in the end, in order to keep piece and still punish, sanction or state something important, the USA prefers to use embargos since they're still considered a diplomatic measure.
