A.
The terms used in the Dangerous Goods Regulation are explained in Appendix A. I will attempt to explain using the definition listed in Appendix A. I will first show the English text from Appendix A, followed by my comments.

DANGEROUS GOODS
Articles or substances which are capable of posing a risk to health, safety, property or the environment and which are shown in the list of dangerous goods in these Regulations or which are classified according to the Regulations.
(Source: DGR 1.0; DGR Appendix A)
[Comments: Therefore non-dangerous goods are defined to be articles and substances not conforming to the afore-mentioned definition of dangerous goods.]

APPROVAL
An authorization granted by the appropriate national authority for:
a) the transport of dangerous goods forbidden on passenger and/or cargo aircraft where these Regulations state that goods may be carried with an approval; or
b ) other purposes as provided for in these Regulations.
Note:
In the absence of a specific reference in these Regulations allowing the granting of an approval, an exemption may be sought
(Source: DGR Appendix A)
[Comments: Please refer to DGR 1.2.5.1. Actual examples are those approved under SP A1 and A2 where you may ship an item which may not be allowed in normal circumstances.]

EXEMPTION
Authorization, other than an approval, granted by an appropriate national authority providing relief from the provisions of these Regulations. The requirements for exemption are given in 1.2.6.
(Source: DGR Appendix A)
[Comments: As stated in DGR 1.2.6, at time of a massive earthquake or tsunami, there will be a need to rush relief supplies. States may grant exemption from following the rules.]

EXCEPTION
A provision in these Regulations which excludes a specific item of dangerous goods from the requirements normally applicable to that item.
(Source: DGR Appendix A)
[Comments: DGR 1.2.7 lists all such items of Exception. Such as medical aid to passengers on board, or humane killer to an animal in flight or spraying chemicals from the aircraft for various causes.]

NOT RESTRICTED
Means not subject to or restricted by these Regulations, except as otherwise stated.
(Source: DGR Appendix A)
[Comments: Refer to Barium sulphate listed in 4.2 DG List. It is a chemical but is Not Restricted with no additional condition. Whereas, for Batteries, dry, also shown in the DG List, although it is marked Not Restricted you are obliged to meet SP A123 to be qualified as Not Restricted.