Josh Samuelson has a fantastic LinkedIn Learning course about Terraform and answers this question quite clearly:

• Resources are the building blocks of Terraform code.
• Resources define the 'what' of your infrastructure and Terraform figures out the 'how'.
• Although all resources share the same syntax, every provider has different resource types and slightly different options.
• A 'provider' isn't a resource. It's the thing that gives you access TO resources. For example, a resource could be AWS.