From Rigpa Wiki
The Nyingtik (Wyl. snying thig or snying tig) teachings are the innermost secret teachings of Dzogchen. The Dzogchen teachings were revealed to Prahevajra (Tib. Garab Dorje) by Vajrasattva, and passed down through an unbroken lineage to present day masters. Within the Dzogchen teachings, there are three categories of teachings suitable to students of different capacity. The Nyingtik is the innermost secret unsurpassed cycle of teachings (Wyl. yang gsang bla na med pa'i skor) of the Category of Pith Instructions; this cycle is the most direct approach for students of the highest capacity.
- The instructional teachings are elucidated and condensed in two major traditions of Nyingtik.
- The first one is the detailed teachings for/of the scholars, brought to Tibet by Vimalamitra and known as Vima Nyingtik. It is mainly based on the Seventeen Tantras and the Troma tantra.
- The second one is the profound teachings for/of mendicants [or yogis], brought to Tibet by Guru Padmasambhava and known as Khandro Nyingtik. It is mainly based on the Longsal Barma tantra. 
Tulku Thondup continues:
- In addition to the original tantras of Nyingtik, the innermost esoteric teachings of Mengakdé translated into Tibetan, many other Nyingtik teachings were brought to Tibet mainly through Vimalamitra and Guru Rinpoche (and also Vairotsana). Some were transmitted in pure visions to great realized masters by siddhas who lived centuries before them. Some were discovered by masters in their state of realization of primordial wisdom. Some were revealed by masters as ters, which had been transmitted to them in their past lives by enlightened masters and awakened in this life to be revealed by others. They include
- Vima Nyingtik ['old' Nyingtik & kama lineage]
- Khandro Nyingtik ['new' Nyingtik & terma lineage]
- Chetsun Nyingtik [a reminiscence of Jamyang Khyentse Wangpo]
- Gongpa Zangthal [revealed by Rigdzin Gödem]
- Kadak Rangjung Rangshar
- Gongpa Yangdü
- Yangtik Nakpo
- Ati Zabdön Nyingpo
- Karma Nyingtik [written down by the Third Karmapa, Rangjung Dorje]
- Longchen Nyingtik [revealed by Jikmé Lingpa]
- Ösal Nyingtik
- Dorsem Nyingtik [revealed by Kunkyong Lingpa in the fifteenth century], and
- Tsogyal Nyingtik.