Najasah means the thing which is unclean in itself, and makes other things unclean by contact. There are ten (10) Najasah:
1 and 2. Urine and stool of those living things whose meat is forbidden (haram), and whose blood comes out with a gush, other than birds. For example the urine of sheep is not najis, because its meat is not haraam though its blood comes out with a gush. Likewise, the stool of snake is not najis because its blood does not come out with a gush, though its meat is haraam.
Note: Urine and stool of those animals are najis which eat refuse of the human beings or have drunk milk of pig.
3 and 4. Blood and semen of those living things whose blood comes out with a gush, whether they be halal or haraam. So all these things of human beings are najis, and also those of cow are najis.
5. Dead body of those living things whose blood comes out with a gush. But there are some details: Those parts which have no life in them during life-time, like hair, nail, bone, teeth etc. are not najis. The corpse of a Muslim is najis after being cold and before being washed. Before being cold it is tahir; after being washed it is tahir. The limbs which are cut from a living body (as in operation) are treated as corpse. (Ivory is taken out from dead elephant; still it is not najis, because it is a part which has no life in the life-time of the elephant (You may think of other examples yourself).
6. The dog.
7. The pig.
9. Liquor (liquid intoxicant).
10. Fuqqa (mild beer).
Note: The sweat that comes out during or after becoming unlawfully 'junub', and the sweat of the animal which eats refuse of human beings are not najis; but prayer with such sweat on body or clothes is not allowed.