Permissions for This Document Library (or List) Setting
Use the Permissions for This Library (or List) to create unique permissions for the library/list in SharePoint 2010 (rather than inheriting the site-level permissions). Follow these steps to create unique permissions:
Choose the Permission for This Document Library (or List) link from the Customize page.
You see a message bar below the Ribbon that tells you that the library/list is inheriting from the parent site. You can stop inheriting to apply unique permissions. If you've already broken inheritance on the list, you can revert to inheriting from the parent again.
Click the Stop Inheriting link.
The users and groups that have access to the parent (in this case, the site) become available to delete or modify. Make sure that you leave owners with the ability to edit the list!
Think through this process carefully. Breaking inheritance often causes confusion for both users and site owners. Users who can access everything on a site can be baffled when they're denied access to a certain library or list.
Site owners who may have at first liked the idea of granular permissions are suddenly stuck hunting down what's causing access problems. Consider using a subsite with different permissions to contain exclusive libraries or lists.
Use the Permissions Tools tab on the ribbon to modify permissions by clicking the option buttons.
One of the common scenarios in this situation is the need to remove certain entire site groups, for example, visitors (readers), and add specific users to the list.