The phrase "e pluribus unum" is Latin, and it translates literally as "out of many, one." Many people are familiar with this phrase from the context of the Great Seal of the United States, which appears on some American currency and government documents. Several organizations have also adopted the phrase as their mottos, both in the United States and abroad.
This phrase has murky origins that are difficult to pin down. A similar phrase appears in the works of Virgil, specifically in a recipe for salad, or at least so claims Bill Bryson in Made in America, a survey of American English. The term was also used by Gentlemen's Magazine, a popular publication among the upper classes in the 18th century. Every year, the publication would print an issue featuring the best work from the previous year, and undoubtedly the upper class Americans in the government of the nascent nation would have been familiar with the magazine and this annual "best of" issue.
Whatever the origins, "e pluribus unum" was adopted by Congress in 1782 as an official motto, along with "annuit coeptis" ("he has approved our undertakings") and "novus ordo seclorum" ("a new order for the ages"). Almost immediately, the phrase was being integrated into design proposals for the Great Seal of the United States, and by 1795, it was also appearing on American currency. Incidentally, American currency was not widely standardized until the middle of the 19th century; prior to this period, banks freely printed their own currency, and shopkeepers also had to accept foreign currency, keeping a formidable table of exchange rates in their heads.
On the Great Seal of the United States, the phrase appears in the banner held in the beak of the American eagle. The busy eagle is also holding an olive branch and a quiver of arrows in its left and right talons, respectively. The phrase is meant to symbolize the union of the 13 original colonies, and their close relationship with the federal government. Over time, people have also taken "e pluribus unum" to refer to the ethnic diversity in the United States.
The motto appears on all modern American currency, sometimes along with the eagle and, more often, independently. The Great Seal of the United States is also featured on American passports, government documents, bills that have been signed into law, treaties, and other formal communications from the United States government. Incidentally, the Secretary of State is the keeper of the official physical seal of the United States, which is used over 2,000 times a year on official documents.