Lists are static in nature, whereas Segments are dynamic. The criteria set on the List will fetch only the existing contacts to the List. Whereas, the criteria set on the Segment will fetch both the existing and newly added contacts.

To expand further, after you have created a List, only the existing Contacts that meet the criteria specified in the List will be added to it. Whereas, both existing and new Contacts you create after the Segment has been created will be added to it.