In data analytics we use several types of segmentation: practically a segment is a subset of your Analytics data.
In web analytics - according to Google - for example, of your entire set of users, one segment might be users from a particular country or city. Another segment might be users who purchase a particular line of products or who visit a specific part of your site.
Segments let you isolate and analyze those subsets of data so you can examine and respond to the component trends in your business. For example, if you find that users from a particular geographic region are no longer purchasing a line of products in the same volume as they normally have, you can see whether a competing business is offering the same types of products at lower prices. If that turned out to be the case, you could respond by offering a loyalty discount to those users that undercuts your competitor's prices.
You can also use segments as the basis for audiences. For example, you might create a segment of users who visit your menswear pages, and then target just those users (your audience) with a re-marketing campaign that is focused on the new items that you are adding to those pages.
Segmentation usually helps to answer specific business questions and / or can be leveraged in making better business decisions and marketing.
Cheers,
Chill