When creating a team site, you have two options : create the site with unique or a inherited permissions.

If you create a site with unique permissions, the link between this site and the site it was created under (the parent site) is broken.  You can have different permissions on both sites.

If you create a site with same permissions as parent site, it means it is inherited permissions.  That means that whatever happens on the site above it affects this site as well.  And vice versa.  If this example below was a collection of team sites, changing permissions in any of the green blocks affects all the green blocks up and down the structure.

Creating sites with inherited permissions is fine for intranet sites – sites that are open to everyone in the organisation.  It’s a different story on team sites – sites that are ring-fenced to a specific department / team.  Only very advanced users can successfully manage team sites using inherited permissions.  The normal business user who has a day job will battle to manage a structure comprised of inherited permissions.  Make sure you plan your sites carefully so as to know which options to choose when.

When selecting unique permissions for a new team site, you will be prompted to create the new groups for the site.  The default groups are Members, Owners and Visitors.  Note what happens to the Visitors group.  It automatically inherits the Visitors group from the site above it even though we have specified unique permissions.  SharePoint is thinking on your behalf and assuming you want all the visitors from another site to see this site.  No!  Make sure you click on Create a New Group first!  (If you’ve slipped up and already created the site, and the site is still empty, it’s easier to just delete the site and start again).

When you click Create a New Group, the three group names will all have the same naming convention which matches the name of the site.  You don’t add people to the site now!  What are you adding them to?  You create and build your site first, and add users last – keeping in mind how permissions works so you can structure your site correctly.

With your site created, go back to Site Actions – Site Permissions to view the settings.

You can now manage this site in isolation to the rest of the structure with a separate audience.

• SharePoint 2010 Permissions for Site Owners – Part 2 : Members, Owners, Visitors
• SharePoint 2010 Permissions for Site Owners – Part 3 : Creating a New Group