As an Administrator, you get unlimited access to the organization your user-account is a part of. You can add new devices, edit existing devices, set up Alerts and Integrations, use the Insight reporting tools, etc.
You're also able to manage users in the Account menu as an Administrator - where you can invite new users with a role of your choosing, change roles for existing users, as well as remove users:
As an Editor, you have access to all of the same features as an Administrator, except for the user management part. You can, however view the users of the organization and their user roles:
The Viewer role has a much more limited access, where some of the menus that's available for the other user roles are not available:
In addition to this, it is not possible to edit devices, add new devices or remove devices as a Viewer. Even if the Insight reports are not available for this role, they can download sensor data as a CSV-file.