How are segments different from lists?
A list is a static collection of people’s email addresses grouped under an identifiable label. You can add people to a list manually, or people can join a list themselves by subscribing through a form. Segments are defined by a set of conditions rather than a fixed list of people. A segment will grow as new people meet the conditions and shrink as people no longer meet them.
You would use a segment to send targeted emails, trigger email flows, or for your own analysis. If you need ideas for the types of segments to make, check out this article on creating customer engagement tiers.