They are essentially the same, but British English spells some words differently (words like "color" and "favorite" are spelled "colour" and "favourite", and some words like "utilize" are spelled with an s--"utilise"). British English also uses some words differently- in America, we generally say "elevator", while in Britain they say "lift". Here's a page with some examples- http://www.bellaonline.com/articles/art40165.asp

If you speak American English, you shouldn't really have any trouble understanding British English unless they use some slang that you don't recognize.