A segment is a subset of your Analytics data. Segments let you isolate and analyze those subsets of data so you can examine and respond to the component trends in your business.
In Google Analytics a segment is a way to specify the data you are seeing in every standard view.
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.
Segmentation is a subset of your Google Analytics data. For example, in your entire user set, a segment might be users from a specific country or city. Another audience segment might be users who purchase a specific product line or visit a specific part of your website.