backward / backwards
The spelling with and without the s.
In American English, the standard form is backward, as an adjective and as an adverb. Backwards is the British form.
Examples
Use The quarterback threw a backward pass to the tailback.
Use The canoe drifted backward into the reeds.
Avoid The quarterback threw a backwards pass to the tailback. (Change backwards to backward.)
Avoid The canoe drifted backwards into the reeds. (Change backwards to backward.)