Six classes of tantra

Six classes of tantra (Wyl. rgyud sde drug) — in the Nyingma school of Tibetan Buddhism, the tantras are divided into six classes:

    the three outer classes of tantra (which are common to both the Sarma and Nyingma schools):

    1. the kriya tantra,
    2. the charya tantra and
    3. the yoga tantra.

and

    the three inner classes of tantra (which are specific to the Nyingma school):

    4. the Mahayoga tantras,
    5. the Anuyoga tantras and
    6. the Atiyoga tantras.

Source

www.rigpawiki.org