Rational is the head used for rational numbers.
You can enter a rational number in the form n/m.
The pattern object _Rational can be used to stand for a rational number. It cannot stand for a single integer.
You have to use Numerator and Denominator to extract parts of Rational numbers.
See The Mathematica Book: Section 3.1.1.
See also: Rationals, Integer, Numerator, Denominator.