Writer George Bernard Shaw declared that “England and America are two countries divided by a common language.” Certainly we have different words for the same things; Americans walk on the sidewalk, for example, while we use the pavement. Both countries have their own idiomatic expressions and colloquialisms, but nowhere is the difference more noticeable than in the spelling of words.
Most of the changes to the English language as used in the USA occurred during the nineteenth century and were brought about by the influence of men like Noah Webster (responsible for the Webster dictionaries) and writer Mark Twain. They believed that English spelling should be made easier and should better reflect the actual pronunciation of the words.
They held that words like sulphur, for example, should be simplified to sulfur, while words containing double vowels (ae and oe) would make more sense if written with an ‘e’ only. So, encyclopaedia (British English) became encyclopedia (American English).
These differences in spelling aren’t as well known as that more obvious one, the dropped ‘u’, in which colour becomes color, honour becomes honor, labour becomes labor, and so on.
In America the simplification of English spelling led to other changes designed to reflect the actual pronunciation of words. So, words ending in ‘re’ became words ending in ‘er’, – center rather than centre, for example – while words such as defence and offence changed to defense and offense.
This account is, of course, a brief and incomplete one and anyone wishing to learn more about the differences in spelling would be advised to consult an American-English dictionary.
It is also worth noting that the word America itself refers to the American continents (North, South and Central America) and not to the United States alone.