SharePoint Permissions... Part 4
Breaking Inheritance

The owner has the permission to create unique permissions at the time you create a site. If you change your mind later, you have to break inheritance.
To stop inheriting permissions from a parent site:
1.    Browse to the People and Groups page for a site
2.    Click the site permissions header in the Quick Launch bar.
3.    Choose Actions > Edit Permissions – a dialog box confirming that you wish to stop inheriting permissions appears.
4.    Click OK – the site permission levels and SP groups are no longer Read-Only.
To re-inherit permissions from the parent site: Actions > Inherit Permissions. Any changes you’ve made are discarded, and the site inherits the parent’s permissions.
Note: When adding users to a group at the list or library level, you’re actually adding users to the entire site collection group. Individual lists and libraries don’t have their own SharePoint groups

Audiences do not have anything to do with security. Audience is a group to which content is targeted. The users in the audience are the only users to see it (because of filters) the user becomes part of a specific audience based on his profile properties, groups etc. audience content targeting has nothing to do with access. Therefore, if a user does not have the permission to view an item, does not mean that he/she cannot have access to the item.