Every ExpressionEngine website has a frontend website (such as the Agile Records website you can see at http://localhost/)
and a backend control panel (http://localhost/admin.php
or http://localhost/system/index.php).
The control panel is only accessed by you and others who may be responsible for managing the website and creating the content — no one else even needs to know it exists.
Since most of the work that goes into building an ExpressionEngine-powered website will take place in the control panel, it is worth first becoming familiar with how it is organized.
Note, there is also a member control panel, which can be accessed by members of the public (if you so choose). This pared-down control panel allows the member to customize settings specific to him (such as what their member profile page says). Unlike the administrative control panel, the member control panel provides no settings to manage the overall website. You will learn more about the member control...