How the organizational structure works
Initially in your Google Admin console, all your users and devices are placed in a single organizational unit. All settings you make in the Admin console apply to this top-level organization and therefore to all users and devices in your account.
Apply settings to groups of users or devices
To give different settings to a group of users or devices, create an organizational unit beneath your top-level organization. Then move users and devices to that sub-organization. When you make a setting in your Admin console, select the organization. Users or devices in that organization get the new setting. They inherit all other settings from their parent.
In the picture, Gmail, Hangouts, and Drive are turned on for users in the top-level organization. Users in the sub-organization inherit Gmail and Drive being turned on, but for them, Hangouts are turned off.
Apply settings to one user or device
To change settings for a single user or device, create an organization for just that user or device. A user or device belongs to only one organizational unit and inherits that organization's settings.
Build a hierarchy
Create as many organizational units as you want—either at the same level or in a hierarchy. Each child inherits settings from its parent, which you can then customize. Changing a setting at a higher level changes the setting for all sub-organizations that inherit that setting. Custom settings, however, remain unchanged.
If you use multiple domains
You can mix and match users from all your domains in an organizational unit. In fact, users in all your domains are initially in the same top-level organization. To change settings for users in a particular domain, create an organization for just those users.