Segments let you create groups of subscribers.
You can create a segment:
By using filters on the Contact List
By typing in the segment name when importing a new CSV
Types of Segments
When creating a new segment, you can choose to create a Static or Live segment.
A Static Segment captures all of the subscribers that match your filters right now. Subscribers can be manually added or removed from this type of segment. New segments created with CSV imports default to static.
A Live Segment captures all of the subscribers that match your filters right now, as well as subscribers that match your filters in the future. Live segments continuously update as subscribers enter or exit the filters you set.
You can use Live Segments to create a master segment that combines both live and static segments, e.g. a Live Segment of new subscribers from your website and a static segment that you can use to import new subscribers via CSV.