Mitochondrial DNA, maternal DNA or mtDNA is DNA that is passed down from mothers to their sons and daughters. It is only carried through the female line and will continue through daughter inheritance but although a son inherits his mother’s mtDNA, he does not pass it down to his own children. Being that a mother passes her mtDNA to both sons and daughters, this means that both men and women can have their mtDNA tested. Such a test can be used to test your direct maternal lineage – your mother’s mother’s mother etc. See graphic below
Below is a great piece that Mary Anne Pentis put together after learning that her mtDNA test resulted in Haplogroup B2. B2 is one of five Haplogroups found in the indigenous peoples of the Americas. Click on image to enlarge.
Below are some good reads on mtDNA.