dropdown-accesslevelAccess Levels simply determine if an item can be viewed by a certain User Group.  They determine what the User Group can SEE where Permission levels determine what the User Group can DO.

There are four default Access Levels with the Joomla 3 installation. These levels are Guest, Public, Registered and Special.

When you click on one of these Access Levels in the list you will see a list of check boxes followed by the corresponding User Group name. 

Access Levels can be assigned to one or more User Groups.

You may create your own Access Level if you need to fine tune what your users are allowed to see.

Guest: The Guest access level is for users that are NOT logged in.  This can be easily confused with Public, but if a Menu item is set to Public, then users can see it both if they are logged in and if they aren't logged in.  If the user is Registered, he won't see the Guest level web page.  

An example of this might be a web page that is labeled Sign Up.  It could be one of the Menu items in a horizontal navigation section.  Well once a user logs in, the Sign Up page would not be accessible, but a Welcome page that has a Access Level of Registered will be viewable.

Public: Items with this Access Level can be seen by people not logged in and by people that are logged in.  This is the default Access Level for all Menu Items, Articles, Categories and Modules. You do not need to click on any other User Groups on the list if Public is checked.  They will already be able to see them because of the Inherited properties.

Registered: Only users that are logged in can view items that have the Access Level of Registered. Again, you don't need to check any of the child user groups on the list if they are a child of Registered (Author, Editor and Publisher are the child level below Registered). 

Note: If you create a new Access Level and assign the User Group Registered to it, but nothing else, The Public won't be able to see it, but the Super Users won't be able to see it either.  This is because, Super Users are not a child of the Registered User Group. You must select Super User if you want that group to be able to see the item.