I have also put this information on Wikipedia since I found that it is very diffcult to obtain information on the following .
Yajnas in the Vedas
There are 400 Yajnas or fire sacrifices described in the Vedas . Of these, the first is called Aupasana , which is to be performed by all , twice daily . This is in a category by itself . 21 more are compulsory for the Dvijas or "Twiceborn " communities .The 1+21 are called Nitya Karmas . (The 21 complusory yajnas also form part of the 42 compulsory samaskaras or sacraments which are compulsory for all dvijas.) The rest of the yajnas are optional, which are performed as "Kamya Karma " i.e. for particular wishes and benefits. "Nitya" does not mean that all 22 rituals are to performed daily. Only the Aupasana and the Agnihotra are to be performed twice daily, at dawn and dusk. The remaining ones have certain allotted frequencies over the course of the year.
The more complicated the Yajna, the lesser the frequency. The largest ones need to be performed only once in a lifetime. The first seven yajnas are called Paka Yajnas, the second seven Havir Yajnas, and the third seven are called Soma Yajnas. Yajnas such as Putrakameshti (for begetting sons), Ashvamedha (to rule 'the earth'), Rajasuya (royal consecration), etc. are among the 400 which are not compulsory.
The Grhyagni or Aupasanagni is used in the Paka Yajnas; such rituals are described in the Grhyasutras, such as in the Ekagni Kanda of the Apastambha Sutra. Normally this fire is located in the north of the hall which accommodates the sacred fires . This fire may be circular or square .The rituals pertaining to the three Srautagnis are described in the Shrauta Sutras. Their performers are called Srautin. Fourteen of the 21 compulsory sacrifices are performed in the Srautagnis. They are called Garhapatya, Ahavaniya and Dakshinagni and collectively called the tretagni. The Garhapatya is circular in shape and is situated in the west of the offering ground. Fire is taken from the Garhapatya ( which is to be kept alive throughout ones life )and kindled in the remaining two fires as and when required for a particular sacrifice. The Dakshinagni is semicircular (halfmoon-shaped), is situated in the south and is used for certain rituals, mainly for offerings to the forefathers. The Ahavaniya is square, situated in the east, and is used as the main offering fire of most Srauta sacrifices. The last three Havir Yagnas and all the seven Somayajnas are performed in a specially built Yajnashala, since they are too elaborate to be performed at home . In such a situation, fire is taken from the Garhapatya , kindled in the Ahavaniya as required and then from there transferred to the specially built Altar at the Yajnashala.The individual's last rites and cremation are done with fire taken from the Dakshinagni , after which the sacred fires are extinguished .
Pakayajnas are minor sacrifices and are performed at home in the aupasanagni or grhyagni . These are seven in number .They are Ashtaka , Sthalipaka , Parvana , Sravani , Agrahayani , Chaitri , Ashvayuji. The sthalipaka is to be performed on every Prathama (first day of the lunar fortnight) "Sthali" is the pot in which rice is cooked; it must be placed on the aupasana fire and the rice called "Charu" cooked in it must be offered into the same fire. The Parvana is to be performed every month . The other five are to be performed once a year .
The haviryajnas are more elaborate (with the exception of the Agnihotra), though not as large in scale as the somayajnas. The haviryajna performed on every Prathama day ( every fifteen days )is "darsa-purna-isti", "darsa" meaning the new moon and "purna" the full moon. The two rituals are also referred to merely as "isti". The Darsapaurnamasa isti is the prakrti (archetype) for the haviryajnas. The first four haviryajnas - adhana, agnihotra, darsa-purna-masa and agrayana - are performed at home. The last three haviryajnas - caturmasya, nirudhapasubandha and sautramani - are performed in a yajnasala.The Agnihotra is to be performed twice daily at sunrise and sunset immediately after the aupasana .Though it is a Havir Yajna it is quite simple .The other five Havir yagnas are to be performed once a year , or at least once in a life time .The last two havir yagnas have animal sacrifice ( traditionally goats) as part of the ritual . However nowadays , packets of flour or butter etc are used as symbolic substitutes .
The name somayajna is called after the juice of the Soma plant, said to be relished by the devas, that is offered as an oblation. In these sacrifices, Samans from the SamaVeda are sung, and all Shrauta priests - the hotar, adhvaryu, udgatar and the brahman as well as their 12 helpers take part: each priest is assisted by three others. with a total complement of 16 priests . The Agnistoma, the first of the seven somayajnas is the prakrti (archetype) for the six others that are its vikrti. These six are: atyagnistoma, uktya, sodasi, vajapeya, atiratra and aptoryama. Vajapeya is often regarded as particularly important. When its yajamana (sacrificer) comes after the ritual bath (avabhrtha snana) at the conclusion of the sacrifice, the king himself holds up a white umbrella for him. "Vaja" means 'prize of a race' (but is nowadays also taken as rice, food) and "peya" means a drink, thus 'drink of victory'. This sacrifice consists of the offering of soma-rasa (juice), pasu-homa (offering of 23 animals) and anna- or vaja-homa. The sacrificer is "bathed" in the rice that is left over. Since the rice is "poured over" him like water, the term "vajapeya" is apt.Animal sacrifices are part of the ritual offerings in the soma yagnas . However , nowadays as and when they are rarely performed , substitutes made of flour butter etc are used instead of sacrificing live animals .
The Vedic sacrifices begain to decline soon after the Islamic rule in India began in the 13th Century ., especially the more public and elaborate and expensive ones .Today , a few of the basic ones are kept alive on important occasions like Weddings and so forth .However , by and large ,the Vedic Sacrifices are close to extinction . of nearly 200 million hindu families , the Aupasana is performed daily only in a few thousand families .Less than a hundred families perform the remaining 21 sacrifices as required and keep the sacred fires continously alive . Priests who are qualified to guide one through all the 21 sacrifices number less than a few dozen . Priests or scholars to guide one through any of the remaining 378 Vedic Sacrifices can be counted on ones fingers .It is a great loss to the Sanathana Dharma if ultimately no one remembers how these were performed .