A fraction is a part of a whole. There are two numbers to every fraction:
The top number of the fraction is called the numerator. The bottom number is called the denominator.
To find out the fraction all you need to know is how many parts there are. For example, in the picture above there are 7 boxes, which is the number the rectangle has been divided into. Therefore the numbers of parts there are is the denominator. Also in the picture above 2 of the boxes are shaded, this means the number 2 is the numerator, which is the parts you have. So denominator is what something has been divided into and the numerator is the number of parts you have.