What's the Relationship Between CiviCRM Contacts and CMS (Drupal/Joomla) Users?
Information on synchronizing Drupal/Joomla "users" to CiviCRM "contacts" is at Synchronize Users-to-Contacts.
A good approach for integration with Drupal users is to create a single CiviCRM profile with all the fields that you want to collect from your users. Then check the New User Registration and User Account checkboxes in the profile settings. This way, each registering Drupal user will be prompted to fill in the fields you want to collect from them and they will be able to modify the information from the Drupal "My Account" link.