American English | British English
------- | --------
color | colour
neighbor | neighbour
center | centre
analyze | analyse
defense | defence
catalog | catalogue
theater | theatre
practice | practise
tire | tyre
judgment | judgement
license | licence
realize | realise
organize | organise
traveler | traveller
canceled | cancelled
acknowledged | acknowledged (can be either)
program | programme
Other differences:
* -ize vs. -ise: American English generally prefers "-ize" endings (realize, organize), while British English uses "-ise" (realise, organise).
* -or vs. -our: American English uses "-or" (color, humor), while British English uses "-our" (colour, humour).
* -er vs. -re: American English uses "-er" (center, theater), while British English uses "-re" (centre, theatre).
* -ing vs. -re: American English uses "-ing" in the present participle (traveling), while British English sometimes uses "-re" (travelling).
Important Note: These are just a few examples. There are many other words with slightly different spellings between the two varieties of English.
It's always best to be aware of the context and audience when choosing spellings, especially in formal writing.