What are the fundamental differences between Shi`ahs & Sunnis authentic notions? Whether the differences are basic and threatening to faith (Iman) or just trifle?

There are a number of differences in the Shi`ah (Shiite) and the Sunni Juristic opinions. However, not all these differences can be termed as the "basic differences" in these two major sects of Islam. The basic difference in the two sects is that of the Shiite faith of the system of "Imamah". The Shiite faith of "Imamah" implies that after the Prophet  (pbuh), there shall be no other prophet, but the only true leader of the Muslims, at any given time, is an "Imam" who, like the prophets of God, is directly appointed by God. The appointment of the first "Imam" was made by God through the last Prophet (pbuh), while every subsequent "Imam" is appointed through the "Imam", who precedes him. Another qualification of the "Imam", according to the Shiite belief is that he shall belong to the family of the last Prophet (pbuh). The Shiite belief holds that the "Imams", like the prophets of God, are "ma`soom" (sinless, innocent) and, therefore, should be obeyed in all matters and under all circumstances. The "Imams", according to the Shiite faith, are thus not just the political leaders of the Muslims but also their religious leaders and clergy. The Sunni school, on the other hand, does not ascribe to any such belief.

This may, at first sight, seem to be a trivial difference between the two schools. However, a close analysis reveals that it amounts to a difference in the basic sources of religion and religious knowledge information and directives for the two schools. The Sunni school, because of its lack of belief in the institution of "Imamah" holds the last Prophet of God and the book revealed on him as the two primary sources of Islam, while the Shiite school, because of the importance and position it gives to the "Imams" holds them to be an autonomous source of their religion. Anything that an "Imam" says, anything that he does and anything that he narrates is "religion" for the Shiite school. Differing with an "Imam" in any matter is of about the same consequence as differing with a prophet. Not submitting to the directions of an "Imam" is as grave a sin as refusing to submit to the directions of a prophet of God.

As can be seen from the above explanation, the difference between the Shiite and the Sunni schools are not trivial. On the contrary, these differences are "very" basic. Nevertheless, the thing to do is not to condemn each other; or call each other "Kaafir" (infidel) or non-Muslim"; or air the fire of hatred and promote each other's killings, but to try and convince the other person as well as one's own self to accept whatever is right in the light of the Qur'an  and the true teachings of the Prophet  (pbuh), to promote empathy for each other's points of view and to try and stress the common points in each other's beliefs.

It should be interesting to note here that although these two sects have co-existed for centuries, yet the extent and gravity of sectarian violence has never been felt more than during the present times. The basic reason for this change in the situation is that a number of prominent scholars and knowledgeable personalities of both these sects have changed the roles that they played in the past. In the past, the differences of both the sects were a matter of intellectual debate between the scholars of these sects. The gravity of these differences was also fully known, but the scholars never promoted violence against each other. The situation has tremendously changed during the past few decades. The scholars of these sects now sit on their respective pulpits and use these differences as flames to burn the emotions and feelings of their addressees against the people of the other sects. They promote the killings of the followers of other sects by labeling these followers with various derogatory terms. They call such killings a part of Jihaad and they promise their followers the everlasting bliss of jannah (paradise) if they take part in such killings.

...Yes, I do agree that the difference in the Shiite and the Sunni sects are quite basic and not trivial, but I think there is still no need to fire each other's emotions to such great extents, even on differences that are so grave.