MAIN DIFFERENCES BETWEEN AMERICAN AND BRITISH ENGLISH SPELLINGS (from askoxford.com)
(a) British English words that end in -re (e.g. centre, fibre, theatre) often end in -er in American English (center, fiber, theater).
(b) British English words that end in -our (e.g. colour, humour) usually end with -or in American English (color, humor).
(c) Verbs in British English that can be spelled with either -ize or -ise at the end (e.g. recognize/recognise) are always spelled with -ize in American English.
(d) Verbs in British English that end in -yse (e.g. analyse) are always spelled -yze in American English (analyze).
(e) In British spelling, verbs ending in a vowel plus l double the l when adding endings that begin with a vowel (e.g. travel, travelled, traveller). In American English the l is not doubled (travel, traveled, traveler).
(f) British English words that are spelled with the double vowels ae or oe (e.g. archaeology, manoeuvre) are just spelled with an e in American English (archeology, maneuver).
(g) Some nouns that end with -ence in British English (e.g. licence, defence) are spelled -ense in American English (license, defense).
(h) Some nouns that end with -ogue in British English (e.g. dialogue) end with -og in American English (dialog).