An Instance is the front end, or customer-facing view, of the SAP SuccessFactors system platform. SAP SuccessFactors customers work with implementation consultants to configure their instance of SAP SuccessFactors.
Consultants, administrators, and end users see the same interface, but just like any other system, access to tools varies depending on what has been configured.
Provisioning is the key configuration tool SAP SuccessFactors uses to control many aspects of the customer instance. In essence, Provisioning is the back end of the system. Customers never use Provisioning, nor do they have access to it.