What is Homam/Havan
The rules for Vedic fire offerings have come down to us in two parallel systems. A few of the later exegetical passages (brahmana/pandit/purohith) in the Vedas refer to cooked food (paakaa) offerings contrasted with the multiple-fire ritual otherwise being prescribed. Later Smarta tradition, the Brahmanical tradition that adheres to the secondary revelation, smrti characterizes the full priestly cult as based on the revealed sruti, sacred texts of the Vedas hence srauta, in contradistinction to the homely-grhya or worldly laukika, or even popular in the sense of conventional.
General rules found in several gruhya ritual manuals (Gruhyasutras) specify several ways in which a homa employing a cooked offering (paakayagna) can be distinguished from a srauta offering. The Varaha opens with a definition of the pakayagnas: they are offered in the domestic fire; they follow the model of the srauta new-and-full-moon service/omitting the fore-offerings and after-offerings, the offerings to the wives of he gods, and the kindling-verse offerings samidheni. On the other hand, jaimini suggests that they may optionally follow a format of their own darsapoornamasatantrah svatantra va. The mantras accompanying the actual oblations conclude with the exclamation svaahaa, Varaha-Gruhyasutra. At most, one priest is to be employed, the brahman, while the householder himself performs the function of the hotra. The daily Morning and Evening offerings dispense even with the brahman priest.
The Manava-Gruhyasutra makes some general observations about the general features of and occasions for Vedic domestic ceremonies:
in the case of groups [of rites] a simple grass strew, the fire, the aagaraas, aagyabhaagas, aahutis, and the svistakrt are the common elements. In the annual [and] fortnightly rites, [and] in the worship preceding harnessing, circling [the field], sowing, and harvesting, the furrow-worship, the threshing-floor worship, the sacrificial tether worship, and the draft ox worship, he worships the deities Agni, Indra, Soma, Sita, Savitri, the twin Asvins, Anumati, Revati, Raka, Pusan and Rudra. He worships Varuna in rites at rivers, pools, wells, and tanks; Soma in rites involving plants and trees; Agni in all rites.