区 (qū) is a general noun for 'area, region, district' — it can refer to an administrative division (like a city district) or any defined zone. It is more specific than 地区 (dìqū), which is broader and often geographical/political, and less formal than 区域 (qūyù), which emphasizes a bounded or functional zone. 小区 (xiǎoqū) is a specific compound meaning 'residential community'.