The word backward can be used both as an adjective and as an adverb.
- Take two steps backward. (Here the word backward is used as an adverb.)
- She gave the cottage a sad backward glance. (Here backward is used as an adjective because it goes before the noun glance.)
- We watched the backward flow of the tide.
Backwards can be a bit problematic. It can only be used as an adverb.
- He glanced backward. / He glanced backwards.
- Can you turn the clock backward / backwards?
- He took a backward glance. (NOT He took a backwards glance. Backwards is an adverb. It is not possible here because we need an adjective before a noun.)
If you are not sure about your grammar, always use ‘backward’ and be safe. The same rule applies to words like forward, toward, westward, northward etc. While forward and toward can be used both as an adjective and an adverb, forwards and towards can only be used as an adverb.