Sin Offering

The sin offering was an offering that had to be brought to atone for unintentionally committed sins. It was not a voluntary or spontaneous offering.
The rules for this type of offering can be found in Leviticus 4–5.

When was this Offering Brought?

The sin offering served primarily to atone for unintentionally committed sins. For this offering, there were different rules for the high priest, the whole community, the leader of the people and the ordinary people.
Sin offerings were also required:

to consecrate the altar (Leviticus 8:15);
after the birth of a child (Leviticus 12).

These two examples show that there does not necessarily need to be any sin for a sin offering.

What Kind of Animals were Offered?

The sin offering was a bull, a male goat or a female goat, without any defects.
The animal must be led to the entrance of the Tent of the LORD’s presence. There, the person who wanted to bring the offering must place his hands on the head of the animal. Then he must kill the animal. The priest carried out a number of rituals with the animal’s blood. Finally, the priest burnt the animal on the altar. The skin, the innards and the meat were burned outside the camp.
If several offerings were brought, the sin offering was always the first.