What is the difference between segments and lists?

Both result in a group of recipients, but they are maintained in completely different ways, and that is where the difference lies.

List — a snapshot

A list is a collection of contacts someone has selected. It is accurate on the day it is created and starts becoming inaccurate the next day. People change jobs, customers stop being customers, and new ones come along. After six months, you are sending to a reality that no longer exists.

Segment — a rule

A segment defines who should be included: all contacts at customers in a specific industry who have made a purchase within the past two years and have given their consent. The system continuously calculates membership. New contacts who meet the criteria are added, while those who no longer do are removed.

When a list is still the right choice

For one-off sends to a group that cannot be defined by a rule—for example, attendees at a specific event. The rule is simple: if the send is recurring, use a segment.