Static Group vs Criteria-based Group
Segmentation group can be one memorizing particular persons (static group), or one memorizing your selection criteria (criteria-based group). Criteria-based groups are smart enough to automatically include any lately add contacts which meet their criteria.
Response Based Segmentation
How about grouping all people who click on the "Learn More" button in an email marketing campaign? You know these are the real potential buyers and we help you grouping them easily for future marketing activities.
A group can be used for campaign targeting which can also be used for exclusion. How about sending to all your database contacts but excluding those who purchased before for a newbie special offer?