Before 1960 most scientists used the metric cgs units, centimeters, grams, and seconds. In addition many other units were derived from combinations of these units, e.g. force (dyne), energy (erg). However, for a complex set of motives, it was decided in 1960 to standardize on a world metric system called "Systeme international d'unites", or just SI, in which the basic units are meters, kilograms, and seconds.
It may not seem much of a change, after all the cgs and the SI are still basically the same metric system. However, all the derived units changed size and name. The SI unit of force is the newton, and the SI unit of energy is the joule. All publications now use SI units, but older books and articles may use cgs units, so I thought you would want to know about them.
You might well ask why now we don't use meter, gram, and second as the basic units? Why did we use the centimeter instead of the meter, and why do we now use the kilogram instead of the gram? It's all very complicated, and besides I don't really know.
Another complication is that some of the BI units used in the US and in Britain have drifted apart, e.g. the US gallon and the BI gallon are not the same size. I think this is because the US gallon has 32 ounces while the BI gallon has 40 ounces. The BI system gives me a headache. Another reason to use SI units.