1. In Hinduism, sūtras seem to have originated as manuals for those concerned with household and other rituals. Sūtra literature is written in a condensed prose. The Kalpasūtras are concerned with ritual, and fall into three major categories: Śrautasūtras, Gṛhyasūtras, and Dharmasūtras. As the names imply, the first deal with the performance of sacrifices (in complex detail), the second with home rituals including saṃskāras, and the third with these and with other duties belonging to the āśramas. They were extended in the verse-form śastra literature. Sūtras are also sharp and elliptical works which are commented on in the darśana (philosophical) works: e.g. Jaimini, Bādarāyaṇa, Kaṇāda, Patañjali.
2. In Buddhism, sūtras (Pāli, sutta) are the collections of the discourses or teachings of the Buddha. In Theravāda, they are gathered in the second part of the Pāli canon (tripiṭaka), the Sūtra-(Sutta-) piṭaka. They are then divided into five collections, nikāyas (Skt., āgama). In Mahāyāna, many additional sūtras have been preserved, some of which become foundational for particular schools of Buddhism (e.g. the Lotus Sūtra, Sukhāvatīvyūha, Laṇkāvatāra-Sūtra).
3. For Jain sūtras, see AṄGA.
The Sanskrit word sūtra (Pāli, sutta), or "discourse," is the name generally given to any text said to contain the words or the teaching of the Buddha. Whether or not it actually does is another matter; many sūtras clearly postdate the Buddha's time. Typically, a sūtra begins with the phrase "Thus have I heard," which is presumed by tradition to be the words of the Buddha's attendant Ānanda repeating at the First Council what he heard the Buddha say at a given time and place. The sūtra-piṭaka (basket of discourses) represents one of three major divisions of the Buddhist canon (Tripiṭaka), the others being the vinaya and the abhidharma.
John S. Strong