After more research I believe I can answer the question myself . I appreciate any feedback on my findings:
When a user opens Tableau Desktop he will select Tableau Server as datasource. He then specifies the server name. If Tableau Server is configured to use Active Directory, behind the scenes Tableau Server redirects Tableau Desktop to the Active Directory Provider. The user is then prompted for his AD credentials*. After the user enters the credentials, he is authenticated on the server and can perform all actions such as connecting to different data sources on the server and opening and publishing workbooks.