American English versus British English

This is too big a topic to answer in a forum post. Wikipedia has at least ten pages devoted to discussing various elements of the topic. A good starting point is http://en.wikipedia.org/wiki/Comparison_of_American_and_British_English

To start, please understand that British English and American English, especially in their written form, vary little in their essential form. Compared to what they have in common, the differences are minor. Nonetheless, there are differences in pronunciation, spelling, grammar (notably preposition usage), and vocabulary (especially idiomatic and colloquial usages).

The story of BrE and AmE is one of divergence followed by convergence. AmE represents the grammatical structures and, to a lesser degree, pronunciations of BrE as spoken in 1625. Isolated in North America, AmE “froze” while BrE continued to evolve. After about 200 years of relative stasis, AmE changed rapidly during the 19th century, bringing about further divergence from BrE. Spelling changed. Noah Webster, author of the first popular American dictionary, “rationalized” many spellings in the belief the new American nation’s language should be more rational and scientific. Pronunciation changed. The first generations of school textbooks taught spelling and vocabulary through rote exercises. Students were required to sound out each syllable and then spell it. The caused AmE to sound out elements of words that are silent in BrE. Lancastershire has 3 syllables in BrE, but 4 in AmE. New words entered the AmE vocabulary as Americans borrowed words from the native languages of the European immigrants, from the Native Americans, and from their Spanish neighbors. Other new words were invented to describe the conditions and fit the lives of the pioneers as they moved into the Western wilderness.

Many vocabulary differences can be traced to new technologies that developed between 1850 and 1950. For railroads/railways, BrE and AmE independently developed different words for the same things – sleepers/ties, points/switch, and so on. For the automobile, we have bonnet/hood, boot/trunk, and so on.

Since World War II, the story has become one of convergence. Globalization in all forms—media, commercial trade, higher education, research, and political influence—has meant greater interaction between all forms of global English. All variants of English are evolving due to this stimulus, but the dominant position of America (for better or worse) has resulted in BrE gradually acquiring features of AmE in recent years.  While all English dialects generate new words, AmE has become the single greatest source of new global words, due in large measure to its leadership position in science and technology and the popularizing effect of American media.

Read more on Wikipedia.