Ratios compare quantities of the same kind by division and are used in many real-life examples like mixing cement and sand, idly batter, juice proportions, and ratios in the human body. A ratio expresses the relative sizes of two quantities using a fraction format and can be simplified to its lowest terms, as seen in examples like simplifying the ratio 24:48. Ratios are also used to divide quantities, such as sharing 750 in the ratio of 5:10.