RP and General American

The topic I have chosen is very interesting, I suppose. It is about the difference between British and American English. This topic was the only one I paid my attention on from the very beginning. I study this language at school for 10 years. English is an international language spoken in the countries all around the world. Of course, it is very important to speak English and all other languages right. English is different in Great Britain and America, Ireland an Scotland. And I am interested in this difference. I thought English is similar in different countries. Well, it is, but it is as similar as different! What do I mean? My father always speaks English on his work. Once he has been to America and spoke to a man. And during their conversation they didn't understand each other almost. My father said : "That man spoke american, not English." He saw my surprise and explained me that people speak different in America. So I will try to find this difference and find the reason why does it happen.