Good question,I remember a few days ago my little brother asked me the same question.As an english learner I'm gonna try to explain you the difference between them according to my researchs for.
There are many differences among US and British English.First of all into pronunciation:for example the word water,the Americans pronounce it [woder] it's like there was letter D at the place of T.So for them(USA)the T often sounds D but for the Englishmen,water is pronounced[woter]as you see it.Another instance God,for the Americans is pronounced[G^d,or G ^p].Second of all,locutions or expressions and terms;for instance:I already ate(USA) and I have already eaten(UK).I did biology at Harvard(US) and I studied biology at Cambridge(UK).In terms of diplomacy into embassy,the Americans use the verb deny(Es.:Your request to enter in USA is denied),the Englishmen use the verb refuse(Es.:Your request to enter in UK is refused)There are some differences but actually they try to discuss about topic for having Standard English meaning to find a way for making english simple for everyone(US,UK,CND,AUST...) above all for the foreigners.For more infos you can go online wikipedia.com the difference between USandUK english.Thanks,take care,bye.