Drupal 7 provides the ability to add custom fields to the user profile through admin/config/people/accounts/fields.
I have added a firstname field. It appears on the registration page but in the database the field is not added. Where it is stored?
The additional fields are stored in a separate table like field_data_field_first.
When the field is added, a new table is created. In Drupal 6 fields might be added as columns to the main table, but in Drupal 7 that is never the case.