Y-DNA refers to the DNA on the male sex chromosome – the Y chromosome. We each have 23 pairs of chromosomes: 22 pairs of autosomal chromosomes and two sex chromosomes – the X and Y chromosomes.
Females inherit an X chromosomes from each parent (but no Y chromosome), while males inherit an X chromosome from their mother and a Y chromosome from their father. Very little recombination (mixing) occurs between the X and Y chromosomes in males, hence the Y-DNA essentially remains unchanged through the paternal line, providing an incredibly useful way to trace paternal ancestry.