Segmentation is the umbrella term given to subdividing a larger email list into sub-lists based on shared characteristics or qualities.
The purpose of segmentation is to personalize and customize the messages to make it more relevent to specific groups of people.
A study by Mailchimp suggests that segmented campaigns perform significantly better than the non-segmented ones.
Overall, segmenting may also have a positive impact on the email reputation which may improve deliverability.
There are a variety of ways an email list could be segmented, e.g. geographic location, age, gender, expressed interests and purchase history etc.