女子 is formal/literary and used in official contexts: 女子足球 (women's soccer), news reports. In daily speech, use 女人 (nǚrén, woman) or 女的 (nǚde, female, casual).
culture
The classical phrase 巾帼不让须眉 (jīnguó bú ràng xūméi) means 'women are no less than men' — a favorite saying when praising accomplished women.
