Three Types of DNA

Y-DNA.

Only males have the Y chromosome; it is the chromosome that makes a male a male. Y-DNA, which rarely changes (mutates slowly) is passed down to sons from the father's direct paternal/male line - the father's father's father's father's father . . . .
Y-DNA is useful to genealogists because in many cultures the surname of the male passes down each generation along with the Y-DNA information. This allows for connections to be made with others who share a common male ancestor. Female family researchers often obtain DNA from a male relative with the family surname (and Y chromosome) in order to further their genealogical research.
The Y-DNA test can NOT uniquely identify a specific individual. The Y-DNA test used by genealogists is NOT a paternity or forensic/crime test and nor does it reveal any health/medical information.

mtDNA (mitochondrial DNA).

Both males and females have mtDNA. mtDNA passes down from the mother's direct maternal/female line - the mother's mother's mother's mother . . . .
mtDNA is more useful for deep ancestry information (anthrogenealogy) than for genealogy. Occasionally useful genealogical connections are made using mtDNA, but this is the exception rather than the rule, for two reasons:

mtDNA mutates very, very slowly, and
in many cultures, the surname of the mtDNA line is not passed on to each generation because females acquire a new surname when they marry.

The basic mtDNA test used by genealogists does NOT uniquely identify a specific individual. This test is not used for maternity tests, forensic/crime purposes, nor does it reveal any health/medical information. (The full sequence mtDNA could contain health/medical information.)

Autosomal/Chromosomal DNA.

MOST of your DNA is autosomal/chromosomal DNA. Half your autosomal/chromosomal DNA is from each of your parents. Unfortunately for genealogists, this DNA is "all mixed up" (recombined) and cannot be isolated for genealogical information. This is the DNA that can uniquely identify a specific individual. This is the DNA that contains almost all of your health/medical information. This is the DNA used in maternity/paternity tests. This is the DNA used for forensic/crime purposes (the CODIS test). This DNA is NOT used in Y-DNA or mtDNA genetic genealogy projects.

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