backward / backwards
The adjective and the adverb, and the extra s.
Use backward for the adjective: a backward glance, a backward step. For the adverb, backward is the usual American form: move backward. Backwards is also used as an adverb and is the usual British form. Do not use a backwards glance.
Examples
UseShe took a backward step.
UseThe line moved backward.
AvoidShe took a backwards step.