A fraction is used to represent an amount that is not a whole number.
A fraction can be thought of as a part of something.
Look at the following figure:
The circle is divided into four equal parts.
Three of the four parts are shaded.
This can be described using the following fraction:
The bottom number in the fraction is called the denominator.
This is the total number of parts
The top number in the fraction is called the numerator.
This is the partial amount.
For the figure above, we can also use 
to represent the number of parts that are not shaded.