OUs are containing folders for computer and user accounts that are joined to your domain. OUs themselves are managed and manipulated by using the Active Directory Users and Computers tool, and this is the way domain administrators commonly keep all of their objects organized. In a simple environment, you may have an OU for Users and another OU for Computers. Getting a little more advanced may bring you separate OUs for Accounting, Finance, Human Resources, and so on. Taking full advantage of OUs will result in multiple OUs contained within larger-scope OUs. For example, you may have an OU for Accounting user accounts, and a separate OU for Accounting computer accounts. Or you could even create separate OUs for desktop computers versus laptop computers. Maybe one for tablets, one (or many) for your servers... the list goes on and on. If you wanted to get really crazy, you could create a different OU for every single one of your computers! (Please don't do this, as the admin who takes your job after you retire will loathe you because of it.)

Nesting OUs is a very common practice as well. Just like creating folders inside of other folders by using File Explorer, you can use AD Users and Computers to create OUs inside other OUs. This is important for making a clean structure to contain all of your domain objects, but it is also important to the Group Policy processing... er... process.

When you ask any administrator who has worked with Group Policy before, "Where does that GPO apply?" they will almost certainly start thinking in terms of "What OUs does this GPO apply to?" Applying Group Policy at the OU level is our default mentality when working with GPOs, because it is by far the most common tier to which settings are applied. Linking GPOs to particular OUs gives us extreme flexibility in handing different settings to different groups of people or machines. In contrast to the domain-level GPO shown earlier, here is a screenshot of a GPO that is being linked to only one OU (Human Resources). Even though many other OUs exist and contain objects, the settings inside the Firewall Settings GPO will only be applied to those machines that are sitting inside the Human Resources OU: