| Mythic personification of the United States of America. He is classically depicted as a tall, thin, elderly man with white hair and a long goatee. He wears a blue waistcoat, a red bow tie, red-and-white striped pants, and a star-spangled top hat, though his appearance is usually fluid enough to allow for a wide variety of different Uncle Sams in cartoons, posters, and other artwork.
The character's appearance probably has its earliest origins back during the Revolutionary War with a character called Brother Jonathan, who wore a similar costume but had black hair and no goatee.
The name "Uncle Sam" probably got its start during the War of 1812. A man named Elbert Anderson had received a contract to supply troops in New York and New Jersey with provisions. He subcontracted with Ebeneezer and Samuel Wilson, a couple of brothers from Troy, New York, for meat shipping. The casks of meat were stamped "E.A. - U.S." Abbreviating "United States" down to "U.S." was a new and unfamiliar practice for many, and soldiers who received the casks speculated jokingly that the stamp meant that the meat came from either Elbert Anderson or "Uncle" Samuel Wilson. The name caught like wildfire, with Uncle Sam becoming a well-known reference to the government and, by extension, to the American people within the space of only a year or two.
Uncle Sam's appearance really started to gel during the Civil War, when many cartoonists, particularly Thomas Nast and Joseph Keppler, began to draw Sam to resemble the current president, Abraham Lincoln. After Lincoln's death in 1865, the more Lincolnesque Uncle Sam became the default appearance.
But the artist who did the most to transform Uncle Sam from a simple mascot to a full-blown national symbol was James Montgomery Flagg. In 1917, Flagg, inspired by a British recruitment poster of Lord Kitchener commanding Britons to join the Army, used himself as a model and painted the iconic image of a stern Uncle Sam pointing his finger at viewers. "I WANT YOU for U.S. Army" the poster intoned gravely, and that was all that was needed. From that point on, Brother Jonathan got the heave-ho. Columbia moved to a retirement home. Uncle Sam was the only game in town, as far as national personifications go.
Of course, those were the glory days for Sam. After the World Wars, America's increasing cynicism about the government and about the nation itself saw far fewer heroic depictions of the patriotic lovechild of Sam Wilson, Tom Nast, and J.M. Flagg. A Vietnam-era poster featured a bludgeoned and disillusioned Sam pleading "I Want Out." And the standard Sam used in most editorial cartoons nowadays is a far cry from what he used to be -- either pudgy and self-satisfied, obese and grasping, or tattered and beaten.
Still, on our better days, it's nice to think that the classic version of Uncle Sam is out there somewhere, either exhorting Americans to be better at what they do, or searching for the perfect beacon of liberty we were supposed to be...
Research:
"America's Uncle" by Steve Darnall, included in Uncle Sam by Darnall and Alex Ross, 1988, Vertigo Comics (I highly recommend this comic, by the way. Go seek it out, if you can find it.)
Biography of Uncle Sam
The Most Famous Poster
Wikipedia | 
