Joomla! controls access to certain areas and features of a site through the use of Access Control Levels - (ACL).

Using these groups means you can easily control what your visitors and users see and do.

There are four Front-end groups available:
(Front-end means : http://www.yoursite.com)

Registered - This group allows the user to login. Registered users can't contribute content, but this may allow them access to other areas, like a forum or download section.

Author - This group allows a user to post content. They can submit new content, select site-positioning options and select publishing dates. But their content has to be approved before being shown. After that they can even come back and edit their article.

Editor - This group allows a user to post and edit anyone’s content item from the Frontend. They can also edit content that has not been published. Editor users cannot publish or change the status of any articles, not even their own.

Publisher - This group allows a user to post, edit and publish any content item from the Front-end. Publishers can review all articles, edit and change publishing options and they can also decide when an article is to be published, and who it is visible to… (Registered, Author, Unregistered Public)

There are three Administration section groups that allow access to Joomla:

Manager - This group allows a user to access content and other system information from the Backend. Managers are like publishers with Backend access. They don’t have any access to user management or the ability to install components or modules.

Administrator - This group can access most administrative functions. An Administrator user has all the privileges on the back end of a Manager, but they also can install/delete components, modules and bots, have user manager access and can view the site statistics. They cannot change, edit or install Site Templates or make any changes to the site’s global configuration options. When logged in through the Frontend, they are treated as Publishers.

Super Administrator - This group allows access to ALL administration functions and areas. Once created they cannot be deleted – even by another Super Administrator.

THEN, you can extend user groups andd access levels with JACL, read about it here

And one of the best Joomla! Extensions completes the trio - Community Builder