Accents within England and America can be just as different from one another as English accents from American ones. There are different slang words , a few different spellings, but people from each country can understand each other perfectly ( and Australia too). Americans watch British TV and read their books and vice versa so many slang words are understood by both sides.

Imane, Good question. There are an amazing number of differences in pronunciation, vocabulary, and slang. The various accents can be very, very different and sometimes difficult to understand. There's a famous quote: "The United States and England are two countries separated only by a common language." Now, that's not to say that we can't generally understand each other. However, especially when it comes to slang, there are plenty of "British" common words that I, as an American, have no idea what they mean. One example, in the Harry Potter series, is the word "snog". It is never, ever used in the US (except when talking about Harry Potter). However, in England, it is a very common word meaning..."to kiss"! Hope that helps.