Yagya or sacrifice is considered the most important aspect of Vedic religion. The Brahmanas attached to the Vedas give detailed instruction on how to perform sacrifices, what hymns are to be repeated at each stage of the proceedings, what types of altars and sacrificial pits are to be constructed for each kind of sacrifice.
The rituals performed according to these prescribed ways with the various deities in mind and recitation of hymns is actually yagya.
The three main aspects of yagya have been laid down in the sutras of Katyayanas Dravyadevatatyagah, i.e. dravya (material), Devata (deity) and tyaga (sacrifice).