窗户 (chuānghu) is the everyday word for a window in a building—what you open, close, or look out of. It contrasts with 窗 (chuāng), which is more literary and often used in compounds like 车窗 (car window) or 窗外 (outside the window). 窗口 (chuāngkǒu), at HSK6, is a service counter or computer window.