There are two main classes of users: Web Users and System Users. System Users are people using ERPNext in the company. Web users are customers or suppliers (or portal users).
Under User a lot of info can be entered. For the sake of usability the information entered for webs users is minimal: First Name and email.
Important is to realize that the email address is the unique key (ID) identifying the Users.
Users will have access to all modules for which they have role based access. If you want to block certain modules for certain users, un-check the module from the list.