It's possible to learn about family through the Ancestry DNA testing service. I submitted a sample in August, 2015. You get the ethnic breakdown often seen in their ads, as well as information on people in their system with the same DNA. Through their testing, I confirmed most of the information we have collected over several years and eliminated a few errors.
I was hoping to find a group of people to whom I was related that might help us discover more about great-grandmother Lillie's parentage. For me, it worked. The test confirmed we had correctly identified her father and his legitimate children. I share many DNA markers specific to his known descendants who have samples at Ancestry. I have no sharing with his wife's ancestors or close relatives. They do not come up in my results. But, I do share DNA with a family group, not in my tree, that may be the clue to her mother's history.
In the DNA area, there is a section called Circles, that identifies people that share identical DNA at the same locations on their chromosomes. My results put me in 10 Circles. Three of them have 25 or more related people. One has only 4 connections. Ancestry provides an estimation how close the relationship may be based on the analysis. If the administrator of a tree chooses the public option, you can look at their history.