股票 (gǔpiào) refers to a specific tradable stock or share certificate, and is the everyday word for buying and selling individual stocks. In contrast, 股份 (gǔfèn) is more abstract — it denotes a share or portion of ownership in a company (e.g., 持有股份 'hold shares'). 证券 (zhèngquàn) is a broader term covering all financial securities (stocks, bonds, etc.).