Defining roles is about assigning permissions"Drupal permissions control access to content creation, modification and site administration at the application level. Administrators assign permissions to roles, then assign roles to users. The first user ID (uid) of a Drupal site (uid=1) automatically receives all permissions, no matter what role that user belongs to." - from drupal.org/getting-started/before/terminology#permissions to your users. Many of the modules you will enable in your site come with some type of permissions that you can assign to a role.