A dialect of English whose pronunciation is frozen along with that of
Canadian English. While British English had some drastic sound shifts, American and Canadian English pronunciation had only undergone a few minor vowel changes, as well as the changing of some Ts and Ds to alveolar flaps (butter sounds somewhat like "
budder").
Most of the different spellings of American English (which, for all of you elitest Britons out there, are listed in the
OED) developed in
the U.S.'s early years, some of them created by dictionary maker Noah Webster. The differences are comparable to the ones between Brazilian Portuguese and Portuguese Portuguese.
Another interesting fact about American and Canadian English is that both dialects still use the
-ize spelling for words (organize, organization, etc.), while countries outside North America have almost completely dumped it for the newer -ise spelling. However, the OED and
Fowler's Modern English Usage (both of which are decent books of British origin) prefer the -ize spelling. Folks from North America also use the older aluminum spelling instead of the newer aluminium spelling. (Though neither spelling is the original; the original is alumium.)
Americans also refer to the letter Z using the 17th century name "zee" instead of the name "zed" used elsewhere (including in Canada). Rest assured, the name "izzard" is pretty much obsolete.
Sources: Wikipedia and the Concise
Oxford English Dictionary.