Yajna is mass pooja. It can be conducted in any large scale. There are so many rituals to be followed, some even 5000 years old. All the rituals are symbolic and enriched meaning, Usually Yajnas or Yagams are conducted by priests, who never explain to people what they do. They keep the rituals and their meaning secret for their professional safety.
When the minds come together, the result is fast and sure. That is the importance of Yajnas. Various Yajnas are conducted by the Ashram. Saraswathy Yajnam, to get the blessing of the Goddess of Sarswathy to amass knowledge. Pithru Yajna is conducted to thank ancestors during the karkidka vavu.