ExpressionEngine allows two types of members:
1. Members who can log in to your website, but not into the administrative control panel. These members can see any content that you have coded in your templates as member-only as well as perform any actions that you have set to require membership (such as post comments).
2. Members who can log into your website and also the administrative control panel. Depending on the permissions you give these members, they may be able to complete tasks such as creating or modifying channel content, creating or modifying templates, or changing global settings.
If you have an ExpressionEngine-powered website, allowing members is not a requirement. If you do not intend to support members on your website, it is recommended to turn off new member registrations completely (the first option under Members | Preferences in the main menu).
Membership in ExpressionEngine is based on member groups, which are what determine the permissions...