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 two X chromosomes (one from each parent), while males inherit an X chromosome from their mother and a Y chromosome from their father.
Y-DNA is the only DNA which is passed down strictly from father to son along the direct paternal line (from father to son to grandson etc.). The strict patrilineal inheritance pattern of Y-DNA means that your Y-DNA profile is unique to your paternal lineage and shared by all people who descended from the same patrilineal ancestral lineage as you. Testing your Y-DNA allows you to trace your direct paternal ancestry (your father’s, father’s, father’s… paternal line).