Northwestward means towards the northwest. You can use it as an adverb (they sailed northwestward) or as an adjective (a northwestward path). It is a little more formal or old-fashioned than just saying to the northwest, but the meaning is plain: the direction you face or move, not the place you already are. American English likes this form without the s; British English often prefers northwestwards with the s for the adverb. There is no extra meaning in that choice — it is a spelling habit. Like other -ward words, it pairs with verbs of movement: turn, head, drift, look.

向西北、往西北