Shared Services allows you to provision users/groups to products such as Shared Services, HFM, Workspace, FDM, etc, etc, etc.
In addition to provisioning roles to users/groups; however, you still will perform security management in individual products.
For instance, in Shared Services, I can grant user johndoe rights to access an FDM application. From FDM, you would manage which locations the user can access, etc.
Shared Services also gives you other functionality such as security reports and copying an entire application's security/provisioning to another app. (i.e. Create a new HFM app and then use Shared Services to copy all security provisioning from an existing app to it)