Tags can be used to categorize contacts.
Difference between Tags and Segments
Tags needs to be manually set while segments are automatically assigned based on preset conditions.
Bulk Add/Remove Contacts to Tag
You can upload CSV file with a single email column to bulk add/remove contacts from a tag.
Go to contact tags page. Locate tag and click Upload CSV.
Specify if you want to Add Contacts or Remove Contacts. Attach CSV file with single email column and click Upload.
Keep in mind that CSV upload will only add/remove existing contacts from tag. It will not create new contacts and will skip suppressed contacts.
Tag in Workflows
Tags can be used triggers to start a workflow whenever a contact is added. There's also workflow action that allows you to add/remove contacts from tags.