Behavior segmentation is the first step of creating a tailored email campaign. It is the process of filtering a customer database (using a CRM) by behavior in order to reach a specific customer segment. Then the campaign is customized to react to or build on that behavior.
This is necessary because the campaign makes sense only to the people who fit certain criteria - for example, sending a cart abandonment campaign to people who have completed an order is useless and hurts their experience.
Behavior segmentation makes your email marketing more relevant, more personalized and therefore, more effective.
Some examples of behavior segmentation: