Hey Mahmoud, thanks for the question!
Sitecore Cortex is more than just the auto-tagging, it is an umbrella brand to cover any intelligent features introduced into the system. Also, it's primary goal is to enable you to use external ML Services (or ML services you have built) and then apply those into a Sitecore context.
For example, the Cortex Auto Tagging feature is a completely plugin-driven module that allows you to integrate any natural language provider that you use (or your own) and then provides the capabilities to take the results of that call and leverage it within the Sitecore content tree.
Sitecore provides the default providers for this framework, integrating with Open Calais as the NLP, but then you can mix and match your own customizations to extend it to do what you need to. For example, SXA has a different provider set because it does a few different things with the results from the NLP for different fields.
Beyond auto-tagging, you will also see Cortex primarily around personalization suggestions and the new processing engine.
Sitecore AI is a different thing altogether. Those are SaaS-based services that will deliver you specific functionality (sort of like OpenCalais acts as an NLP). The only available Sitecore AI service right now is auto-personalization, with more coming.
You can read a bit more about that on the Sitecore blog: www.sitecore.com/.../sitecore-symposium-day-3-first-look-at-sitecore-ai-auto-personalization