Vegetables — drumstick, aubergine, okra, shallots — cooked in, and a tempering to finish.
Where it comes from
A popular story says sambar was first made at the Maratha court of Thanjavur and named after Sambhaji, the Maratha ruler — though it's probably a legend, and sour lentil stews were surely made in the south long before.
Every household has its own sambar powder, and every region its own sambar: sharper in Tamil Nadu, sweeter with jaggery in Udupi, with ground coconut in Kerala.
How it varies
Arachuvitta sambar
With spices and coconut roasted and ground fresh rather than from a powder.
Udupi sambar
Sweetened with jaggery, and often with ground coconut.