Once you start sending campaigns, the system collect statistics about your contacts and labels a User type for each contact.
You can use this data for optimizing campaigns by segmenting and sending only to specific user types.
User types:
Free - People who did not perform any action
Clickers - People who clicked on a message before
If you integrated conversion tracking, you will see additional user types:
Leads - People that are marked as Leads
Buyers - People that are marked as Buyers