The "metric system" is the standard system of weights and measures developed in France more than 200 years ago and now used throughout the world. It is a decimal system that uses consistent terms to indicate how units relate to each other. For example, the "meter" is the standard unit of length/distance and the "gram" is the standard unit of weight/mass. Add the prefix "kilo-" and you've multiplied the basic unit by 1,000. A kilometer is 1,000 meters and a kilogram is 1,000 grams.
The "English" system of weights and measures is the older system formerly used in the British Empire, but now superseded by the metric system — except in the United States. The English system standardized relationships between many traditional units, such as the foot, yard, and mile (for length and distance), the ounce, pound, and ton (for weight), and the (liquid) ounce, pint, and gallon (for volume). Thus the names of units are traditional and the relationships between them are arbitrary and difficult to remember. Because it is not a decimal system, the English system is more difficult to work with. While it's easy to remember that there are 1,000 meters in a kilometer, almost no one can tell you how many yards are in a mile.