British and American spelling
There are several areas in which British and American spelling are different. The differences often come about because British English has tended to keep the spelling of words it has absorbed from other languages (e.g. French), while American English has adapted the spelling to reflect the way that the words actually sound when they're spoken.
If you're writing for British readers, you should only use British spellings. In one or two cases, the preferred American spellings are acceptable in British English as well, especially the -ize/-ization endings. While you can use both the -ise/-isation or the -ize/ization endings in British English, it's important to stick to one style or the other throughout the same piece of writing.