There may be occasions where you would like to target contacts in your database that do not have any Tags associated with their contact profiles (i.e.: they are untagged). You can do this by creating an Audience. *
As your contacts have tags added or removed from their profiles, they will be dynamically included or excluded from the Audience.

To create, start by navigating to the Audiences menu.
From here:

• Select Create Audience
• Give your new Audience a name, e.g.: "Contacts with no tags", then select Continue
• Under Filters, select the Select new filter dropdown to see the options available.
• Choose Tags
  
  • Set the condition to does not have any tags
• Select Apply Filters to see how many contacts currently match this filter set.
  
• Finally, select Save Audience

* Users can create an Audience that includes (their matching) contacts. Admins can create an Audience in the same way (without contacts), and share it to their Users as a Managed Audience.