The numerator is the part of a fraction which tells how many parts of a whole there are. It is the number above the line in a fraction.
The denominator is the number below the fraction line. It tells into how many equal parts the whole has been divided.
For example, when dividing an object into thirds, the fraction representing that object has a denominator of three. Its numerator depends on how many of the divided pieces the fraction represents. If it represents all three of them, the numerator is three and the fraction is three-thirds. If it expresses only one of the pieces, the numerator is one and the fraction is one-third.