The Nyingma organises the whole of Buddhism into a ladder of nine vehicles, and it is the only Tibetan school that went most of its history without a central hierarch.
The ladder
Three vehicles of the sutra path, three outer tantras, and three inner tantras, ending with Atiyoga, which is Dzogchen. The scheme is not a ranking of sects but a claim about method: each vehicle is complete for those it suits, and the higher ones work with what the lower ones renounce. Only the Nyingma uses this ninefold arrangement, and its top three classes are not recognised as such by the schools that received their tantras in the later transmission.
Householders, not only monks
A large part of the tradition is carried by ngakpa, non-monastic tantric practitioners who marry, wear white and red rather than robes, keep their hair long, and hold transmissions in family lines. In the other schools authority runs through the monastic colleges. Here it runs through both, and the household lineages have preserved material the monasteries did not.
An office invented in exile
There was no Nyingma head until the twentieth century. Monasteries such as Mindrolling, Dorje Drak and Kathok operated independently, and the tradition was held together by shared texts and teachers rather than by administration. The position of supreme head was created after 1959, in exile, largely so that the school could be represented in dealings with other schools and with governments. It is a modern answer to a modern problem, not an ancient office.