Difference between WhatId and WhoId in Events and Task

WhatId:

ID of a related Account, Opportunity, Campaign, Case, or custom object. Label is Opportunity/Account ID.

WhoId:

ID of a related Contact or Lead. If the WhoId refers to a lead, then the WhatId field must be empty. Label is Contact/Lead ID. If Shared Activities is enabled, this is the ID of a related Lead or primary Contact.

Task and Event Objects Relationships: