Ratio is a comparison between to quantities. Ratios can also be written in fraction form. Ratios, like fractions, must always be expressed in simplest form. Simplifying a ratio is the same as reducing fraction in lowest term.
When the values of the quantities being compared change proportionally in the same direction, the relationship is a direct proportion. This means, when the value of one quantity also increases, the value of the other quantity also increases. In the same way as when one quantity decreases, the other quantity decreases also.
A proportion wherein the values of the quantities being compared changes proportionally in different directions is called indirect or inverse proportion. It means that when one quantity increases, the other decreases.