Answer:
The MKS (metre, kilogram, second) system of units is the fundamental core of the SI (Systeme Internationale, or International System) of measurement. As various fields grow new forms of measurements based on MKS, terms or exact definitions may vary in different scientific or engineering disciplines. These inconsistencies are sorted out by the international committee that defines the SI system of units and keeps it up to date.
So the MKS is the static core, and the SI is the evolving, growing system of metric measurement.
Metres and kilograms were introduced after the French Revolution around 1790. Seconds were added by the 1830s, building the MKS system. More units came later. By 1948, a need was seen to develop a clear and consistent set of terms for measurement, and this led to 12 years of work, ending with the release of the first version of the SI in 1960.
The SI now has seven base units of measure. It begins with the original three metre, kilogram, and second, and adds the ampere for electricity, candela for light, kelvin for temperature, and mole for amount (number of atoms or molecules) of a substance.
The SI defines how each fundamental unit can be measured or obtained experimentally. These definitions have changed over time. For example, the metre was originally a measure related to the circumference of the Earth, then a set number of waves of a particular wavelength, and now a distance traveled by light in a vacuum in a certain period of time.
The SI also defines modifying terms such as milli-, centi-, and kilo- that are a specific number of each unit.
The SI includes many other derived units, as well.
By standardizing international use of measurements across different fields of science and engineering, the SI reduces confusion that would slow scientific progress and error that can produce engineering catastrophe such as the loss of the Mars Climate Orbiter in 1999 due to a table that should have been in metric units (kilograms and meters) being provided in United States customary units (pounds and inches).*
The Mars Climate Orbiter and its mission were lost due to a failure to use SI units consistently.
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