What's the difference between active and static segments?

Segments (formerly named lists) are grouping of CRM records based on properties, events, or memberships. 

Active segments

Active segments automatically update in real-time. Records will join the segment when they meet your criteria, and will leave it when they no longer meet it. 

For example, an active segment would be valuable when you have an email campaign to a particular subset of your contacts running over time, and always want the segment to update with new contacts who meet the criteria.

Static segments

Opposite to active segments, static segments do not update as new records meet the criteria. Instead, they are a snapshot of a group of records who meet a certain set of requirements when the segment was created and saved. Records can be manually added and removed from static segments. 

A static segment can be helpful to gain an understanding of records that met a certain criteria on a particular date, such as event attendees. They can be ideal for one-time email blasts.