CGS indicates the Centimeter-gram-second System of Units, 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 being widely used. SI has since been introduced to standardize measurement, with its own units.
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.