If you've learnt British English and you're travelling in the States,or if you've learnt American English and you're travelling in Britain,you'll notice some differences.An obvious difference is the accent,but most travellers find that they don't have too many problems with this.There are some grammatical differences,but they shoudn't make it difficult to understand people,or to communicate.That leaves differences in vocabulary,which can cause misunderstandings.Sometimes the difference is only the spelling,for example,in British English 'center','colour',and 'travelled',and in American English 'centre','color',and 'traveled'.But sometimes the word is completely different in British and American English,and it's good idea to be prepared.