"Backward" is generally used as an adjective, which is a part of speech that modifies a noun, and describes an attribute (as if in answer to the question "Which?" or "What kind of?").
"Backwards" is used as an adverb, which is a part of speech that modifies a verb, an adjective, another adverb, or a whole phrase or clause, and gives information about place, time, manner, cause or degree (as if in answer to questions like "Where?", "When?", "How?" "Why?" "How much?").