There are
--MANY
--SMALL
--UNIMPORTANT
differences.
Here are a few examples.
Spelling: British "colour," U.S. "color." British "theatre," U.S. "theater."
Vocabulary: Fuel for a car, British "petrol," U.S. "gasoline" or "gas." Baby "nappies" (British), "diapers" (U.S.)
Grammar and usage: Differences are MOSTLY a question of FREQUENCY. Many "British" usages are correct but RARE U.S. usage and vice versa. British: "I have already eaten." U.S.: "I already ate." British: "He's in hospital," U.S. "He's in THE hospital."
Pronunciation: Jaguar, British "Jag-war," U.S. "Jag-you-are." Garage, British "GARridge," rhymes with "carriage." U.S. "gar-AZH" (i.e. more like the French pronunciation).
For a very thorough article, see
http://en.wikipedia.org/wiki/Comparison_of_American_and_British_English