A yajna centers on a consecrated fire, built in a measured pit or altar, into which a qualified priest pours offerings, clarified butter, grain, or other substances, while reciting Vedic verses that name the offering's destination among the gods. The fire itself is treated as the mouth of the gods and the messenger that carries the offering upward, which is why its construction, orientation, and kindling follow rules set out in the Brahmana and Shrauta literature in far greater technical detail than most other Hindu rites receive.
Purpose ranges enormously by scale: a household may keep a small domestic fire fed with a handful of grain each morning for ordinary wellbeing, while the great public yajnas described in ancient texts, the horse sacrifice among them, were state rituals meant to establish or renew a king's authority and were performed only rarely, at vast expense, with many officiating priests each responsible for a distinct part of the liturgy. The offerings, the mantras, and the number of priests required scale together, so that a domestic fire offering and a royal consecration are recognizably the same rite performed at entirely different registers. Modern practice keeps the domestic and modest public forms in regular use; the largest royal scale rites described in the ancient texts are known today mainly from those texts rather than from continuous performance.