Segments are dynamic collections of Marketing Campaigns contacts grouped, or segmented, based on the criteria you define. Groups can include data you track about them, such as gender, location, birthday, or package type, or how they've engaged with your emails previously. You can create a segment that pulls from ALL CONTACTS or a specific existing list.
Segments are dynamically updated over time as you add customers who meet the criteria of your segment, or as the traits of your contacts change. For example, a segment with the criteria "opened an email within 30 days" will evolve as contacts engage (or don't engage) with your email. Segmentation can be especially useful to ensure that you are targeting the best recipients for your campaign.