Each geometry belongs to a class in the hierarchy shown in Figure 9. As indicated in About geometries, six subclasses in the hierarchy--points, linestrings, polygons, multipoints, multilinestrings, and multipolygons--are instantiable. The superclass and other subclasses are not instantiable.
The ST_GeometryType function takes a geometry and returns the instantiable subclass in the form of a character string. For more information, see ST_GeometryType.
The ST_IsValid function takes a geometry that has been assigned to an ST_Geometry data type. The function returns 1 (TRUE) if the geometry is valid and 0 (FALSE) if the geometry is not valid. For more information, see ST_IsValid.