The american dollar is the official american currency, actually the amercan dollar is the foreign currency with is used in the principal transactions all over the world.
The american dollar history is not easy to explain but it has its origin in the years of Carlos I King of Spain, when he ordered that the territory that is actually called Mexico, had a currency similar to the one used in Europe called Thealer, that because of a pronunciation matter it was called daler by those who used to live there.
Some years later Benjamin Franklin travelled to London to ask permission for issueing currency and so, stop depending on pounds to finance the civil war that was taken place in american land. The proposal was rejected and Franklin decided to finance the war with currency called continental. One the war was over, United States needed a strong currency to be able to stabilize its incipient economy and continental was not able to offer neither security nor support to the new country project.
Alexander Hamilton, the Exchequer Secretary, decided to adopt the mexican daler that became the dollar because of the american pronunciation, this is the was the daler became the american dollar.
The american dollar let the United States become a unified nation, an made the world noticed it.