Altars have been a part of religion for many thousands of years. Most think that altars started as platforms to hold animal or human sacrifices. That certainly is part of it, but not all. In fact, an altar isn’t always a platform.
According to Wikipedia, in can also be just a place where religious ceremonies are held.
The first altar recorded in the Bible was erected by Noah. During the lifetime of Jesus, the famous altar of sacrifice at the temple was used to sacrifice hundreds of birds and animals daily and that is what Jesus was really objecting to when he chased the money changers and animal vendors out of the temple courtyard, and knocked over cages freeing the animals. Alters can be made of earth, stone, metal or wood depending on what they will be used for. The altar at the Temple of Solomon was said to be a stone or earthen structure completely covered with brass. In Christian churches, altars are usually free standing but alters that take the form of a niche in the eastern wall of the church exist as well.
The Catholic church requires that the main altar be “fixed”, that is, attached to the floor so it is essentially part of the building. In many older churches, the altar was build against a wall and the priest stood with his back to the congregation. Since many of those altars cannot be moved, a portable altar or communion table was added when the mass was changed so the priest now faces the congregation. A relic of a dead saint is often hidden within Catholic altars.
The Anglican church had altars build against the east wall after the reformation, but many now have free standing altars.
The Armenian Apostolic church still has it’s alters built against the east wall.
Interestingly, in the Eastern Orthodox churches, the term “altar” refers not to the table itself, but to the area around it, what other churches would call the sanctuary. The table in the Eastern church is usually made of wood.
In Hinduism, altars are shrines to various gods and often have pictures or statues of gods as well as saints and gurus. Small home altars are also common.
In Buddhism, the altar, called a butsudan, is really a cabinet on a stand. Inside are religious images or a sacred scroll.
In summary, while the specifics vary, an altar is generally a place to sacrifice to God or gods. Often this meant sacrificing animals or humans, but now is more about sacrificing ourselfs, our egos, to do the will of God. While most Churches wouldn’t put it that way, essentially, that is what it is really about and that is really the one and only sacrifice God wants from us.
A Coptic altar built into a wall.