Countries that don't use the metric system use imperial units, a legacy system based on ancient measurements. Feet, miles, gallons, quarts, pounds and ounces are all part of the imperial system. This system came from that used in Great Britain, standardized by the Weights and Measures Act of 1824.
The chief drawback of the imperial system is the difficulty of converting units. For example, converting inches to feet requires dividing a number by 12, while converting centimeters to meters merely requires a decimal point shift of two places.
Most countries have adopted the metric system for most measurements. The only countries that still rely on the imperial system are the United States, Myanmar and Liberia.