Hi @david_iux,
There are two type of users.
1) Admin
2) Front-end user [who registered to our website]
Customers > All Customers: From this section, we get all customers list who are registered in our system.
System > All Users: List of all admins users. Users who can access the back-end of Magento.
if issue solved,Click Kudos & Accept as Solution
I hope, I will be able to clarify your confusion here.