The metric system is different from SI or International System of Units in that it has only three base units, whereas the SI has seven base units. Also, the SI is a modern system of measurement that is based on the older metric system.
The metric system of measurement started after the French Revolution in 1670. Devised by Gabriel Mouton, a mathematician, the system comprises of three base units, namely meter, gram and liter, that are determined accurately by using certain standards as a reference. The older metric system, called the CGS system, is based on seconds, grams and centimeters.
In comparison, the SI system of measurement was established officially in 1960 and is similar to the MKS system, a part of the metric system that consists of meter, kilogram and seconds as the three base units that measure length, mass and time. However, the SI system also includes four other base units, namely ampere, Kelvin, mole and candela, that measure electrical current, temperature, amount of substance and luminous intensity, respectively.
In modern times, all other units are constructed using the seven base units of the SI system. Since the MKS system is also a part of the SI system, the modern system of measurement is more correctly called the SI system rather than the metric system.