Denominator[expr] gives the denominator of expr.
Denominator picks out terms which have superficially negative exponents. Numerator picks out all remaining terms.
An exponent is "superficially negative" if it has a negative number as a factor.
The standard representation of rational expressions as products of powers means that you cannot simply use Part to extract denominators.
Denominator can be used on rational numbers.
See The Mathematica Book: Section 1.4.7.
See also: ExpandDenominator, Rationals.