Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled, That war be and is hereby declared to exist between the United Kingdom of Great Britain and Ireland and the dependencies thereof, and the United States of America and their territories; and that the President of the United States is hereby authorized to use the whole land and naval force of the United States to carry the same into effect, and to issue to private armed vessels of the United States commissions or letters of marque and general reprisal, in such form as he shall think proper, and under the seal of the United States, against the vessels, goods, and effects of the government of the said United Kingdom of Great Britain and Ireland, and the subjects thereofInterestingly, if Wikipedia may be believed, Britain did not reciprocate with its own declaration of war.
To Britain, I guess, pesky America's declaration of war was only a sideshow in the larger strategic death struggle with Napoleon. Instead of declaring war, the Brits merely burned Washington:
Image of British cartoon from 1814 obtained from Pitch, The Burning of Washington (pdf)
Image of Tom Freeman painting obtained from White House Historical Association
What did America gain from the War of 1812? Well, we had our first secession controversy: The New England Federalists, whose region was devastated by the loss of British trade, toyed seriously with the notion of leaving the Union. South Carolinian John C. Calhoun was a War Hawk in those days... but he took notes, which he dusted off in 1832, and which his southern successors built on in 1861.
Other than that? We wanted Canada -- but didn't get it. We did get the satisfaction of winning the Battle of New Orleans (fought after a peace treaty had been signed, although the combatants didn't yet know about it). This was the battle that pretty much put Andrew Jackson in line for the (rebuilt) White House. The battle was later celebrated in a 1959 hit (#1 with a bullet?) for Johnny Horton.
Anything else? Well, while being held on a British warship in Baltimore Harbor, Maryland lawyer Francis Scott Key penned a poem that could be sung to the tune of To Anacreon in Heaven, a song that, with Key's new lyrics, is still sung before sporting events today.
For further reading, see The War of 1812: Remember the Raisin!