There are many informative and engaging online resources on the differences between British and American English. Interested readers may wish to read the Wikipedia entries Comparison of American and British English and American and British English spelling differences. The Purdue Owl likewise offers a brief overview of some of key historic drivers of differences between different varieties of English (as well as of differences with other widely spoken varieties of English) and an extensive list of further reading on world Englishes. 

For a light-hearted tour of how American English evolved after crossing the pond, see this transcript of a lecture by Mark Damen, a professor at Utah State University. Also of interest are blogs such as Not One-Off Britishisms, on the seepage of traditional Britishisms into American English, or Separated by a Common Language, on differences between British and American terminology.