Fractions are a specific type of ratio that compare a part to a whole. They are written in the form $\frac{a}{b},$ where the numerator $a$ is the part and the denominator $b$ is the whole. There are many possible ways of reading fractions, but one universal method is saying $a$ over $b.$ It should be noted that $a$ can be greater than or equal to $b.$ When this happens, the fraction is called an improper fraction. Otherwise, the fraction is called a proper fraction. Fractions are also another way to write a division of the numerator by the denominator. $$\begin{array}{c}\frac{18}{9}\iff 18÷9\end{array}$$ A fraction like $\frac{18}{9}$ can be simplified to $\frac{2}{1},$ or just $2.$ It is important to keep in mind that the denominator of a fraction can never be equal to $0$, since the quotient of division by $0$ is always undefined.