Segments are a dynamic grouping of contact details, they hold criteria rather than a list of people, e.g.; "All males in NSW". A Segment will update the number of Contacts it returns in a search each time by running an updated search on the database and picking up all individuals who meet that criteria. The segments you can create are dependent on what data you have collected. A Segment is not defined to just one List it can work across multiple Lists to extract contact details.
You would use Segments if you wanted to extract Contacts relating to a particular criteria within a list. For example; if you want to send a reminder to all contacts who's subscription expires on 4th January 2019.
How to check contacts that are part of a segment:
You can do a query of your contacts who have met the segment you created.
Segments are dynamic fields and any Contacts in the data base will automatically be added to any existing Segments you set up, including new Contacts added to the data base after set up.