Alibaba, Aliexpress and Taobao all belong to Alibaba Group founded by Jack Ma. However, they present major differences which do not make them competitors, for instance:

• Taobao and Tmall focuses mostly on Chinese domestic market (therefore they are available only in Chinese language), while Alibaba and Aliexpress target international customer buying mostly Chinese products from anywhere in the world (therefore both are available in English language).
• Alibaba is a B2B platform aiming at wholesale or large-volume transactions between businesses; Aliexpress is a B2C platform where generally there is no requirement in terms of minimum order.
• Payments on Alibaba and Aliexpress are generally easier (e.g. concluded through PayPal), while payments made on Taobao and Tmall are more complicated for users not based in China.
• Sellers on Taobao and Tmall must be companies legally registered in China, while sellers on Alibaba and Alipay can be based in any country.
• Tmall also has a dedicated section for international brands selling to China via CBEC (Tmall Global), which does not require sellers to be legally registered in China as they can freely open flagship stores on the platform.

Therefore, if you plan to sell your products to Chinese domestic consumers, your best options will be to open stores on Taobao, Tmall, as well as other platforms.