Two answers for you.
1) CGS stands for Computer Graphics Society. it's a global organization for creative digital artists.
2) CSG (just in case you question contained a typo) stands for Constructive Solid Geometry
CSG modeling is a process used to design complex shapes using such simple solid geometric primitives such as cylinder, frustrum, cone, sphere, box, wedge, pyramid, and so on. (These primitives look similar to the primitives for wireframe drawing but differs in that they represent real solid objects having surfaces and internal body.)
Primitives are comined through the use of Boolean operations such as
union
intersection
difference
CSG works well for regular geometric shapes that can be easily created using primitive solids. The size of the primitive is defined by inputting the dimensions of the desired solid. Moreover, some CAD system offers the function of assembly of manyparts generated.
After the model is produced, drawings can be automatically created from any viewpoint. In a Solid Modeling CAD system, sectional views can also be automatically generated from the model. Design analysis functions (e.g. volume, center of mass,moment of inertia, etc.) can be performed from the model's geometric database. Full color shaded pictures can be displayed with the control of light source and direction, and background lighting.