User and Customer accounts are somewhat related, but they are separate records and have different purposes.
User accounts are meant to manage user authentication/permissions and are created when users create user accounts during the checkout process or at some other time.
Customer accounts are created when orders are placed and users are logged in, but never placed an order before or when users are placing orders as guests.
To access your user accounts:
1. Click on the Settings tab.
2. Click on the Users main menu item, then on the Users located on the submenu.
To access your customer accounts:
1. Click on the Shop tab.
2. Click on the main Customer menu item, then click on the Customers located on the submenu.
If a user account exists while the customer account is created, then the customer account is associated with the user account.
If a user places an order as a guest then the order can only be associated with a customer account. If the user creates a user account after the order was placed, then the user will not be able to see the order under the newly created account.