CGS indicates the centimeter-gram-second system, a variant of the metric system developed for physics. The system treats those three units as basic, defining others from them. Variants of the system cover electromagnetic measurement, some units in wide use, and some in niches, including in astrophysics. SI has since been developed as a standard measurement system, which shares some of the basic units, but with an independent set of definitions and differing units for electromagnetism and other specialized purposes.
For example, the gauss is a CGS unit for magnetic flux density that is still in wide use, whereas SI has the tesla, a different unit for the physical property.