If an individual sins unintentionally… (Leviticus 4:27)
A korban chatas (sin offering) was the sacrifice brought after accidentally committing a sin that, if it had been committed intentionally, would be subject to the penalty of kareis (spiritual excision). Most such sins are violations of negative mitzvos; a chatas was not brought for failing to fulfill the two positive mitzvos that carry a penalty of kareis. This is because our verse continues, “to act (against) one of the commandments of God that is not to be done…” An uncircumcised male neglecting to circumcise himself or a person willfully refusing to offer a Passover sacrifice (the two positive mitzvos that carry the penalty of kareis) do not fit the description of performing actions that God said not to do. Rather, they are failure to perform actions He commanded to be done. An unintentional blasphemer also does not bring a korban chatas because his sin was verbal rather than physical. (See Talmud Makkos 13b.)
All told, there are 43 sins for which one might bring a standard korban chatas, 23 of these are arayos (forbidden sexual relationships) and 17 are other sins. Other sins requiring a chatas included passing one’s child through fire for the idol Molech, anointing with the special shemen hamishcha (see Mitzvah #108), eating on Yom Kippur and eating chometz on Passover; a complete list of the 43 sins can be found in Mishneh Torah Hilchos Shegagos 4:1.
The reason for this mitzvah is as we have stated previously: to cultivate the proper emotions in a person. The one who brings the sacrifice should consider himself deserving of such treatment and that it is only through God’s mercy that he may bring an animal in his place. He should be humbled through his offering and this will motivate greater caution in his future actions.
This obligation to bring a korban chatas when appropriate applies to both men and women at a time when the Temple service is in effect. It is discussed in the first chapter of the Talmudic tractate of Kerisos, starting on the first page (2a). It is also discussed in Makkos (13b), Shabbos (68b) and elsewhere. It is codified in the Mishneh Torah in several chapters of Hilchos Shegagos, including the first. It is #69 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos.