Nichiren Buddhism rests on a judgement about the other Buddhist schools, and the judgement is unusually blunt.
Nichiren, who lived from 1222 to 1282, concluded after years of study that the Lotus Sutra contains the complete and final teaching, that the other sutras were provisional, and that practices resting on them (the Pure Land recitation, the esoteric rites, Zen, the monastic disciplines), were not merely inferior but actively harmful in the present age. He said so to the military government of his day, in writing, and connected the disasters then befalling Japan to its patronage of the wrong teachings. He was exiled twice and came close to execution.
The practice that follows is the daimoku: the chanting of Nam-myoho-renge-kyo, the title of the sutra, held to contain the whole of its power. It is performed before the Gohonzon, a mandala of inscribed characters rather than an image. The claim attached is that buddhahood is available to any person in this body and this life, without ordination, without learning, and without the accumulation of merit across lifetimes.
That combination (a simple practice, a universal promise, and an exclusive claim about every other tradition), is why Nichiren Buddhism has been at once the most accessible and the most contentious form of Japanese Buddhism, and why it is the only Buddhist school that has produced a lay movement with a political party attached.
It is also the only major Japanese school to have made the criticism of other Buddhists a devotional act rather than an occasional embarrassment.