Mitochondrial DNA is passed from women to both sons and daughters. Thus, while mtDNA is passed down the maternal line, men have mtDNA but do not pass it their offspring. Mitochondrial DNA can be very useful in genetic genealogy. It it is much much smaller than the DNA found in the cell nucleus (in the chromosomes), but there are many more copies. This makes it useful for archeology and forensics, since it tends to last longer than nuclear DNA.
Full mtDNA sequencing is not very useful at the present for individuals, but as prices drop and more discoveries are made, this will undoubtedly change.
