Genetic Communities™ are groups of AncestryDNA members who are connected through DNA most likely because they descend from a population of common ancestors, even if they no longer live in the area where those ancestors once lived.
For example, some Genetic Communities trace their roots back to groups of people who were isolated geographically. Mountains, rivers, lack of roads, or other barriers made it likely that each new generation would marry someone who lived close to home. Others have their roots in groups who typically married others of the same religion or ethnic group. In each case, these groups came to share a significant amount of DNA. Modern-day descendants who inherited some of that DNA make up Genetic Communities.
Genetic Communities™ are groups of AncestryDNA members who are connected because they share fairly recent ancestors who came from the same region or culture—even though they may have had different ethnic backgrounds. We find these Genetic Communities using genetic connections between groups of AncestryDNA members.
Ethnicity estimates show your ancestry going back hundreds or even thousands of years ago. We calculate an ethnicity estimate by comparing your DNA to the DNA of a carefully selected reference panel. This reference panel is made up of people who have long-standing, documented roots in a specific area.
Both use your DNA to help you learn more about where you’re from and the people of the world you’re connected to.
Source: Ancestry.com access 28 March 2017/DNA