The samskaras cover an individual's entire life-span - "Nisekadi smasanantakam" - from the moment before he is conceived in his mother's womb to the time when his body is offered to Agni. "Niseka" (impregnation) is a rite performed with the sacrificial fire as the witness; and the funeral rites which come last are performed in the fire.
Agni, the sacred fire, must be kept burning throughout a Brahmin's life. The Brahmacarin or bachelor - student must perform the samidadhana everyday. After he is married, with Agni as witness, he becomes a grhastha (householder). He must now perform the aupasana in the fire. For the vanaprastha (forest recluse), there is a sacred fire called "kaksagni". The sannyasin has no sacrament involving the sacred fire: he has the fire of knowledge (jnanagni)in him. His body is not cremated - that is there is no Agni-samskara for it- but interred as a matter of respect. Strictly speaking, it must be cut into four parts and consigned to the four quarters of a forest. There it will be food for birds and beasts. In an inhabited place the severed parts of the body would cause inconvenience to people. That is why they were thrown into the forest. There it would be food for its denizens; if buried it would be manure for the plants. Now over the site of the interment of a sannyasin's body a Brindavana is grown [or built] : this again is done out of respect. At such sites all that is to be done is to plant a bilva or asvattha tree.
All castes have rites to be performed with the sacred fire. During marriage people belongings to all varnas must do aupasana and the fire in which the rite is performed must be preserved throughout. Today, only Parsis seem to keep up such a practice of preserving the fire. Their scripture is called the Zend-Avesta which name must have been derived from the Vedic "Chando-Vasta". Their teacher was Zoroaster [Zarathustra] : this name must have been derived from "Saurastra". Their homeland is Iran (from "Arya"). If the fire kept by them is extinguished at any time they spend a good deal of money in expiatory rites. With us rituals performed in the sacred fire have been on the decline from the turn of the century. The lifestyle of our people has changed. If there is faith, this great treasure (rites performed in the fire) could be preserved. The most important reason for the loss of faith is the present system of education.
This body of ours has to be finally offered in the fire as ahuti (oblation) to the deities. It is treated as a dravya(material for sacrifice) with ghee applied to it before it is offered in the fire. The ceremony is called "dahana-samskara".
For a general background, please see here