mtDNA or mitochondrial DNA is outside the nucleus of the cell, as opposed to the 46 chromosomes (23 pairs), which are located inside the cell nucleus. Inside the cell nucleus, there are 22 pairs of autosomal chromosomes plus the sex chromosomes (the X and the Y). In addition to the other 22 pairs of chromosomes, females inherit two X Chromosomes (one from each parent) and males inherit one X Chromosome (from their mother) and one Y Chromosome (from their father).
mtDNA is passed matrilineally. That means it is only inherited through the female/maternal line. Men do inherit their mother's mtDNA, but they do not pass it down to their children.
Men also inherit their X Chromosome from their mother but, in contrast, they DO pass their X Chromosome to their daughters. However, they do NOT pass their X Chromosome to their sons. Further, the X Chromosome that a male inherits from his mother has genetic material traceable from many more of her ancestors than the mtDNA that he inherits from her. The X Chromosome will also include DNA from some of his mother's PATERNAL ancestral lines, unlike the mtDNA that only traces through her direct maternal ancestral line.
It is complicated, but it will become much clearer to anyone who studies the charts below on X-inheritance patterns and mtDNA inheritance patterns.
Here are some good charts by Jim Turner (firstname.lastname@example.org) illustrating X-inheritance patterns:
In contrast, here is a chart from Angela Cone showing the mtDNA inheritance (on the right side in red only):
As one can see, the numbers of ancestors contributing to the mtDNA is much more limited than that of the X Chromosome. Therefore, X Chromosome testing has the potential of tracing many more ancestors than mtDNA testing.
For further study on this subject, please see Sorenson's animations here and watch both the mtDNA and X Chromosome videos.