1) -or and -our
British English tends to use -our in adjectives,whereas American English uses -or:

For example: colour (GB) and color (US); neighbour (GB) and neighbor (US)

2) -er and -re
Some noun endings are -re in British English, but -er in American English:

For example: centre (GB) and center (US)
3) -ice and -ise
Some words have -ice ending for the noun and -ise ending for the verb in British English. For example: practice (noun) and to practise (verb). However, in American English both noun and verb are spelled practice.
4) -l and -ll
In British English, when a word ends in a single consonant, it is doubled when we add a suffix beginning with a vowel:

For example: traveller; levelled

However, in American English the 'l' is not doubled:

For example: traveler, leveled

5) Other common words
British English and American English have different spellings for certain words:

For example: cheque (GB) - check (US); programme (GB) - program (US)