CLASSES OF HALAL AND HARAM

(1) Things which are naturally unlawful.
(2) Earnings which are unlawful.
(1) Things which are naturally unlawful.

The things which are naturally unlawful are wine, blood, meat of pigs, dead animals, etc.
Naturally lawful

The things fit for eating in the world are of three kinds,
(A) Minerals and Natural Resources
(B) Vegetation
(C) Animals or similiar kind
(A) Minerals and Natural Resources
Minerals are of different kinds. What grows out of earth; is lawful for eating ( and drink) i.e. salt, water etc. except what causes harm. There are things which have got the effect of poison. They are unlawful as they are injurious things.
(B) Vegetation
They are lawful for eating such as vegetable, grains, fruits etc. except those which remove intellect, take life and ruin health. The things which destroy intellect are wine and intoxicants. The things which destroy life are poisons. The things which ruin health are medicines used out of time (out dated or inappropriate) .
(C) Animals or similiar kind
They are of two kind
(a ) What is eatable
(b) What is not eatable.
Birds, beasts and animals in land and water which are fit for eating and which, if sacrificed according to the rules of Shariat, become lawful . What is not sacrificed according to the rules of Shariat and what is dead are unlawful,
However out of dead animals , two kinds are lawful--fish and locusts.
The following animals are lawful according to this rule worms in food-stuffs and fruits, etc. What is not liking to a particular person is Makruh for eating.
Rasulullah (sallallaho alaihe wasallam) said : Immerse a fly if it falls in food.'
If an ant falls in food, it does not become impure. if a portion of flesh of a dead man falls in food, the whole food becomes unlawful. It is not on account of impurity, as man does not become impure after death, but it is out of horror.
Animal (except fish and locusts) if sacrificed according to Shariat, becomes lawful except its blood and what is attached to its impurities. Regarding vegetables, what produces intoxication in unlawful and what removes intellect is unlawful .One drop of an impure thing renders food unlawful.
(2) Earnings which are unlawful.
There are two kind of earning-- what is taken willingly or unwillingly and what comes to the owner spontaneously. What is taken willingly or unwillingly are of two kinds.
(A) One kind is what is taken without the knowledge of the owner, such as minerals underneath the ground.
(B) Another kind is what is taken from the owner himself. The latter is of two kinds.
(a) What is taken by force from him, What is taken by force is again of two kind
(i) what is taken from the maintenance and care of the owner, such as quadrupeds,
(ii) what is taken from him by virtue of power of the ruling authorities, such as Zakat and other economic liabilities
(b) What is taken with his permission. . What is taken with the permission of the owner is of two kinds
(i) What is taken from him in exchange, such as buy and sale, dower, wages,
(ii) what is not taken in exchange, such a gift, waist??.
Thus the things of earnings are of six kinds.
(1) To become owner of the things of which there is no owner such as minerals and natural resources, to make barren land fertile, to gather fuels and woods from jungles, to take water from river, to take grass, etc. To take these things are lawful provided there is no owner of these things.
(2) What is taken by force and what is not prohibited are the properties gained after battle and without actual battle. They are lawful for all Muslims when one-fifth is taken out from the war booties and divided justly among those who are entitled to them. It is unlawful to take booty from those unbelievers with whom there is treaty.
(3) What can lawfully be taken by force inspite of prohibition of the owner, such as Zakat . It can only be taken by the ruling authorities.
(4) What is taken in exchange of things with the consent of the owner is lawful, such as buy and sale transactions.
(5) What is taken simply with permission without exchange of things, such as gifts, will, etc, is lawful.
(6) What comes spontaneously in possessions, such as properties by inheritance after deduction of necessary expense such as funeral expenses, death instructions by will or otherwise, expenses of expiation of religious duties, expenses of pilgrimage, etc. They are lawful.
Source: Ihya-Ulum-Id- Din by Imam Ghazzali (r.a.) Revival of Religious Learnings-
English Translation by Fazlul Karim Publisher Darul-Ishaat Karachi Pakistan)