Basically, this word refers to excrement or filth which prevents one from a valid salat service. Contact with it would bring a person into a state of ritual impurity. It requires one to undergo purification process before performing religious obligations and duties for e.g salat services.
Different school of jurisprudence have their own system on classifying the condition that a person would be regarded as impure. I am quite familiar with Shafi'ie school of jurisprudence classification on what kind of najasa or najis that requires different way of cleaning them. Shafi'ie school is not so much different to Hanbali school of jurisprudence in classifying the najasa. Types of najas would be subject to the studies of hadith in all school of jurisprudence.
In our school curriculum generally based from the Shafi'ie jurisprudence, I found that we have three types of najasa. This basically comes from a systematic book of imam al-Nawawi rahimahullah known as al-Minhaj. Then, they are followed with the method to clean them. These are the basic ones for Sunnah Muslims and they are....
This refers to the impurity caused only by the pissing of little baby boy below two years old and only depends on his mother breasting feed. In order to clean it whether it messed the floor where people is going to perform worship or on the cloth that people use to perform worship, just remove the filth by wiping it and then splash some clean water on it. Then dry the place or wipe it clean.
This refer to something which comes out from human body or animals but in liquid form. Once it comes out of a human body, he has to perform ablution, no need for full dipping into the water. The e.g are...
a. All types of poops or feces except of the dog and pig as both animals are included in the list to come.
d. Pissing except of the dog and swine as these animals are in the list to come.
f. Bile liquid.
g. Milk from non-halal animals which consist of several different kinds of animals mentioned in Quran.
h. The madzi or wadi liquid (please refer to specific teachers for definition).
To clean these, remove the excrement or filth from the infected area and wash it until the smell, color and the feel are not there no more.
This refer to subject matters which is najas in all of the cases. They are...
a. Dog, its offspring and every part of the animal.
b. Pig, its offspring and every part of the animal.
To clean ourselves or area contaminated with the mentioned subject matters, first of all remove the najas for example if our cloth was contaminated with pig's feces or dogs' licking by wiping them clean with other cloth or toilet paper which would be thrown away. Then wash with the water mixed with clean soil once. Later, it must be followed by six times splashing of clean water.
Some schools of jurisprudence consider semen as ritually impure. It is ritually impure if a person is ejaculating the subject matter and one needs to perform ritual dipping. But it is not ritually impure for both Hanbali and Shafi'ie jurisprudence if one unintentionally touched the subject matter or if it sticks at ones' cloth. Just enough by washing it with clean water. One does not have to perform the ablution as it does not invalidate his ablution only having a contact with it. The evidence for this as according to Imam Nawawi of Shafi'ie school comes from the hadith reports by Imam Ali r.a, Sa'ad bin Abi Waqas r.a and our lady 'Aisha r.a.
2. Dead Body
For some school of jurisprudence such as Hanbali school there is an opinion that the dead body of a non-Muslim would cause one to face ritual impurity or najas. This is a minority opinion by Ibnu Qudama al-Hanbali. Other than that, Shi'a Muslims and some minority of Muslims like myself also consider dead body as ritually impure whether of Muslim or non-Muslim but also with terms and conditions which I am lazy to state here as people are free to follow their school of jurisprudence as long as they are "recognized" by the majority and makes our life easy.
Twelver Shi'a Muslims and the Zaydi Shi'a Muslims who are closer to the Kufa Twelver Shi'a Muslims consider non-Muslims will cause them ritual impurity. However, there are some terms and conditions related to this subject matter where some of the Twelver Imami Marjas consider people of the scriptures such as the Jews and Christians or even the Zoroastrians who possess scriptures as clean and ritually clean. For Shi'a Muslims, they can get the help from their own Marjas or jurisprudence scholars. I have several books and notes about Shi'a jurisprudence too but I am not a Shi'a. This is only for the conduct of respect when visiting Shi'a Muslim brothers' mosques as I sometimes walked by their area like in Thailand.
For a Hanbali Sunnah Muslims like myself, everyone are human-being. So, we can touch or having contact with them without having to perform ablution or performing ghusl after the conduct. The conditions are just the same like those applied to Muslim fellows.
Opinion of People of the Books
As for people of the books, I personally consider Orthodox Indian Brahmins as resembling the people of the books. They posses the Vedas as according to their clans' adherence and Puranas just like Zoroastrians of Persia who posses Avestan scriptures. They also perform dipping in the water almost the same like us. Regarding civilization, Middle East Civilization is far more earlier in its development than Hindu Civilization and I am not surprised with parallel conducts within our traditions or customs. I read Hindu scriptures as well as I studied history subjects while I was a form six student. I also believe in the prophethood of prophet Noah a.s, the ancestor of all nations.
Those mentioned methods and the classification are just the studies of scholars. It is not by any way intended to make ones life in a burden but to make our life easy. So, if there are some differences here and there. Please be tolerant and do not force what we had learned on others. Just respect our fellow Muslim brothers as long as we have basis on every deeds. If there are no basis, just tell them but if they do not want to listen just do not argue or quarrel. It is not the manner to be a follower of Muhammad s.a.w and his traditions.
This post is just a basic reminder of a fellow Muslim to himself and to others who stepped by. It is not a complete lesson from a qualified teacher! To know in detail, there are specific teachers for this topic of cleanliness. Never stop at one point once we already know but further it as we are trying to reach perfection. Cleanliness is among the branches of the faith.
Sealed with prayers for mercy, peace and love, amin!