implementation of STEP

I want to build a reader for STEP file.
Where can I get more informations about STEP schema?
For example I want to know the details of the following STEP entities,rules and types,
ENTITY face_based_surface_model;
ENTITY face_bound;
ENTITY face_outer_bound;
ENTITY face_surface;
ENTITY faceted_brep;
ENTITY faceted_brep_shape_representation;

RULE product_definition_for_class_notation;
RULE product_definition_for_design_still_water_shear_force_values;
RULE product_definition_for_expiry_date;
RULE product_definition_for_flag_state;

TYPE electric_current_measure;
TYPE external_identification_item;
TYPE founded_item_select;
TYPE geometric_set_select;