Re: So what's the difference ...
A corporate version that is (more) compliant with regulations.
WhatsApp is not GDPR compliant, so can't be used on business devices or devices with company data (BYOD) in Europe, for example, and Skype is not a business solution.
Teams takes care of things like logging and archiving etc. that are legal requirements in many business areas and ensures the data remains compliant with local laws (if you ignore the bit about it having to reside in Europe or under equivalent protection and the US are still dragging their feet with getting Privacy Shield off the ground, for European users).