The top number in a fraction is called a numerator. The bottom number is called a denominator. The line that separates them can be horizontal or vertical.
The numerator represents the number of parts and the denominator represents how many parts consist of the whole. It is possible for the numerator to be larger than the denominator, but fractions that represent a value of greater than one are "improper." When the numerator exceeds the denominator the fraction needs to be broken down further.
The denominator of a fraction can never be zero. However, a numerator can be zero and the resulting calculation returns a value of zero.