Spelling
Here are some general differences between British and American spellings:
Words ending in -or (American) -our (British) color, colour, humor, humour, flavor, flavour etc.
Words ending in -ize (American) -ise (British) recognize, recognise, patronize, patronise etc.
The best way to make sure that you are being consistent in your spelling is to use the spell check on your word processor (if you are using the computer of course) and choose which variety of English you would like. As you can see, there are really very few differences between standard British English and standard American English. However, the largest difference is probably that of the choice of vocabulary and pronunciation. For further information concerning these areas please refer to the following links below.
Also from Wiki
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Historical origins
In the early 18th century, English spelling was not standardized. Differences became noticeable after the publishing of influential dictionaries. Current British English spellings follow, for the most part, those of Samuel Johnson's Dictionary of the English Language (1755), whereas many American English spellings follow Noah Webster's An American Dictionary of the English Language of 1828.
Webster was a strong proponent of spelling reform for reasons both philological and nationalistic. Many spelling changes proposed in the US by Webster himself, and in the early 20th century by the Simplified Spelling Board, never caught on. Among the advocates of spelling reform in England, the influences of those who preferred the Norman (or Anglo-French) spellings of certain words proved decisive. Subsequent spelling adjustments in the UK had little effect on present-day US spelling, and vice versa. While in many cases American English deviated in the 19th century from mainstream British spelling, on the other hand it has also often retained older forms.
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Latin-derived spellings
-our, -or
Most words ending in unstressed -our in the United Kingdom
(e.g., colour, flavour, honour, armour, rumour) end in -or in the United States (i.e., color, flavor, honor, armor, rumor). Where the vowel is unreduced, this does not occur: contour, paramour, troubadour, are spelled thus everywhere. Most words of this category derive from Latin non-agent nouns having nominative -or; the first such borrowings into English were from early Old French and the ending was -or or -ur.[19] After the Norman Conquest, the termination became -our in Anglo-French in an attempt to represent the Old French pronunciation of words ending in -or,[20] though color has been used occasionally in English since the fifteenth century.[21] The -our ending was not only retained in English borrowings from Anglo-French, but also applied to earlier French borrowings.[22] After the Renaissance, some such borrowings from Latin were taken up with their original -or termination; many words once ending in -our (for example, chancellour and governour) now end in -or everywhere. Many words of the -our/-or group do not have a Latin counterpart; for example, armo(u)r, behavio(u)r, harbo(u)r, neighbo(u)r; also arbo(u)r meaning "shelter", though senses "tree" and "tool" are always arbor, a false cognate of the other word. Some 16th and early 17th century British scholars indeed insisted that -or be used for words of Latin origin (e.g. color[21]) and -our for French loans; but in many cases the etymology was not completely clear, and therefore some scholars advocated -or only and others -our only.[23]
Webster's 1828 dictionary featured only -or and is generally given much of the credit for the adoption of this form in the US. By contrast, Dr Johnson's 1755 dictionary used the -our spelling for all words still so spelled in Britain, as well as for emperour, errour, governour, horrour, tenour, terrour, and tremour, where the u has since been dropped. Johnson, unlike Webster, was not an advocate of spelling reform, but selected the version best-derived, as he saw it, from among the variations in his sources: he favoured French over Latin spellings because, as he put it, "the French generally supplied us."[24] Those English speakers who began to move across the Atlantic would have taken these habits with them and H L Mencken makes the point that, "honor appears in the Declaration of Independence, but it seems to have got there rather by accident than by design. In Jefferson’s original draft it is spelled honour. "[25] Examples such as color, flavor, behavior, harbor, or neighbor scarcely appear in the Old Bailey's court records from the 17th and 18th century, whereas examples of their -our counterparts are numbered in thousands.[26] One notable exception is honor: honor and honour were equally frequent down to the 17th century,[27] Honor still is, in the UK, the normal spelling as a person's name.