The Y-Chromosome is passed from father to son essentially unchanged. Testing and analysis techniques on various locations (loci) of the Y-chomosome give numerical markers (alleles). A person is related to another individual if the markers are almost identical. Two individuals are placed in the same family group if they are an exact or close match on 25 or more markers. We have identified several distinct family groups. If your marker values match one of the family groups you are related to that family. If your DNA is significantly different from another individual you know that you do not share a common ancestor. A negative result trims the family tree.  You may also view results at our web site which includes individuals who tested at other labs:
http://web.utk.edu/~corn/parkerdna/