When you are using both Drupal and CiviCRM, it can get a little confusing as to what each of the things relating to a person in your site/database means.
Role - Drupal - this is a category that is assigned to a person so that a set of permissions can be assigned to them. Examples include: admin, staff, and editor.
Account - Drupal - an account is what someone has if they have registered on your Drupal site with a username and password.
Contact - CiviCRM - this is a record within your database. It could be an individual (person) contact, organizational contact, or household contact. These are automatically created anytime someone creates an account on your site, fills out a CiviCRM form, donates, becomes a member, etc.
Membership - CiviCRM - this is for memberships within your organization. You can use a module to sync memberships and roles so that the membership can also control permissions over in Drupal.