Tags are used to categorise contacts to optimise casting database search results. Unlimited Tags can be added to to contact profiles.
Tags can be included in the batch Contact Import template dowloadable in Contacts/Import Contacts/download template:
Import Template: 
Tags can also be added directly to contact profiles:
Contact Profile: 
Tags (and Notes) do not trigger GDPR workflows for contacts entered into GDPR processing, nor are they (or Notes) visible in the Contact Interface.
FAQ: