There are a few subtle differences not only in the spelling and pronunciation of certain words but also in what we call certain things. For example, the words 'color' and 'theater' in American English are spelled 'colour' and 'theatre' in British English. Just as 'pants' in American English can sometimes be called 'knickers' in British English. But again, they're not huge differences and if you go to the UK and speak American English, 99% of the time they will understand and vise versa. Did that help?

The intonation can be a big difference with American English. Sometimes everything sounds like a question to the British ear. There are a few spelling differences as someone mentioned. But really only a few. And also some words. Amusing ones: Fanny - America = Buttocks, England = Vagina. Fag - America = Homosexual, England = Cigarette. Colloquialisms would be the big thing that varies a lot. But when it comes to standard conversational English everything is pretty much equal.