X-chromosome – The sex chromosome associated with female characteristics in mammals. It occurs as a pair in females and singly in males.
Y-chromosome – The sex chromosome that is contributed by a father to his son. Y-chromosomes are only present in males.
Y-STR - A Y-STR is a Short Tandem Repeat (STR) found on the Y-chromosome. In a Y-STR test, multiple Y-STR's are examined in order to determine the Y-STR found in the tested male. Usually this same Y-STR profile is found in all other men from the same paternal lineage as the tested male.