Hi @NigelRS , thanks for posting on this community!
I see two topics here:
1. Lists
You can create lists of contacts, and then group them into folders, like this:
Could that fit your need?
2. Tagging contacts
We don’t have (not yet!) any tagging system for contacts, but you can use contact attributes to differentiate your contacts, see more details here: https://help.brevo.com/hc/en-us/articles/10582214160274-About-contact-attributes
Hope this helps!