From the ancient times yagnya had been a traditional practice in the gurukuls where the acharya would speak the knowledge and the students & the kings would listen to that knowledge for the well being and to overcome the abnormalities of the life. Acharya here refers to the Teacher who has perfect behaviour (aachran) & pure brahmnishth knowledge and the ones who are receiving the knowledge are the yajmaan, which means those who are receiving the fruits of the yagnya from the acharya.

With all the knowledge lost, yagnya turned into homa because the gurus & gurukuls are extincted. So, today neither there are yagnya in the world nor there are yajmaan. 

Yagnya is more broad term which implies to "pure knowledge offering to initiate the fire which continues (य + ज्ञ)." and, the receiver of the initiation is a yajmaan who uses this fruit in his life to evolve and this is what is called upaaya (उप + आय; earned while being near). This was the system which gurus in the times of kings and gurukuls followed to be near to the people while guiding them on everyday basis, for their daily life affairs specially for the core issues of ones life in order to make them walk the righteous path. And, the yajmaan (king, the disciples & the people) offered & served the guru with clothes, fruits, flowers and a part of their earning (whatever righteous to the yuga) to keep the household running in a smooth manner, which later on was named as dasvandh (10th part of ones earning). 

The system is still prevailing in various parts of Bharat but in a very lost state because of the lost guru-shishya tradition. Because, there are no yagnya but the wish fulfilling inanimate buildings; temples & ashrams, so, there are no yajmaan who need knowledge but fire homa to fulfill ones wishes/desires. Just the conning by the devil system remains. 
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