yajña, yagna, or yagya (Sanksrit: यज्ञ, "worship, prayer, praise; offering, oblation, sacrifice; fire ceremony") comes from the root yaj, "to worship" — is an outer form of worship in which offerings are made to different deities in a prescribed and systematic manner by qualified priests to supplicate them, so that they would assist the worshiper in achieving certain results in life.
The outer aspect of yajna consists of building an altar, generally with bricks, kindling fire using specific types of grass and wood and then pouring into it oblations such as ghee or clarified butter, food grains, sesame seeds, and water to the accompaniment of chanting of sacred verses from the Vedas. The inner or hidden aspect of yajna is known to those who are familiar with the Vedic rituals. The yajna is the means of worshiping the Brahman or ones own Inner Self. In concept, yajna is any work or spiritual practice that is offered as worship to God.
In the Bhagavad Gita Lord Krishna explains that every aspect that is associated with a ritual of sacrifice, the act of offering, the oblation, the sacrificer himself and the sacrificial fire as well is Brahma (4.23). In the subsequent verses He enlists the various types of sacrifices people perform with various objectives in their minds (4.25-4.30) and concludes that sacrifice in the form of knowledge is superior to sacrifice done with material things. In the ninth chapter he declares,
I am kratu (vedic ritual), I am yajna (sacrifice), svadha (offering), ausadham (medicine), mantra (chant), ajyam (ghee), agni (fire), and hutam (burnt offering).
In Chandogya Upanishad, the yagna is compared variously to the world, the God rain, the earth, man and woman.