A lot of companies have several departments and want to limit the sub sites users can view. For example, the IT department should only see the IT sub site, but the HR department may need to be able to see all sub sites. To set up individual security for sub sites you will need to stop the sub sites from inheriting from the parent.
To do this click on the Site Action menu while viewing the sub site, then select Edit in SharePoint Designer.
On the right hand side of the screen you will see the permissions window. Click the button Stop Inheriting.
The button then changes to a new button.
Now click on the new button. This will bring up the Add Permissions window. Enter in the user name or group name in the top text box and click the check user icon on the right. Select the proper group or set up the custom permissions in the bottom section of this window.
Click ok and you have completed adding users to this sub sites. If you have other sub sites you want them to have access to you will need to repeat these steps for each sub site. Any sub sites that you do leave set to inherit from the parent will use the parent permission settings.
If you change your mind and want to set the site back to inherit you can do this with a button at the top of the SharePoint designer. You may need to close the designer and reopen it for it to update.