The dictionaries I've looked in don't distinguish between these two words, backward and backwards (at least when used as adverbs). Is there some real historical, grammatical or regional difference between them?
As an adverb, either word will do: “put the shirt on backward” or “put the shirt on backwards.” However, as an adjective, only “backward” will do: “a backward glance.” When in doubt, use “backward.”
This appears to be from a book "Common Errors in English Usage".
I know that "toward" is considered US and "towards" is considered UK, so this may be the same. (Incidentally I looked in four reference books and didn't find anything about "backward" and "backwards".)
Well there is a very simple difference between backward and backwards. Backward is used in Ameriacn English ;however, backwards is used in British English. Example: Travelling backwards and forwards between London and New York.- Brit term or Travelling backward and forward between London and New York.-- US term
Backwards denotes direction whereas backward means less developed or slow.