Numerator[expr] gives the numerator of expr.
Numerator picks out terms which do not have superficially negative exponents. Denominator picks out the 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 numerators.
Numerator can be used on rational numbers.
See The Mathematica Book: Section 1.4.7.
See also: ExpandNumerator.