Islamic conceptions of atonement for sin
Qur'an teaches that the main way back to God is through genuine repentance (in Arabic "Tawbah" which literally means 'to return'). See Repentance in Islam for further discussions.
Say: "O my Servants who have transgressed against their souls! Despair not of the Mercy of Allah: for Allah forgives all sins: for He is Oft-Forgiving, Most Merciful. Turn ye to our Lord (in repentance) and bow to His (Will), before the Penalty comes on you: after that ye shall not be helped. (Qur'an 39:53-54)
"Verily! Allah Accepts the repentance of those who do evil in ignorance and repent soon afterwards, to them Allah will turn in Mercy, for Allah is Full of Knowledge and Wisdom. And of no effect is the repentance of those who continue to do evil, until death faces one of them and he says "now have I repented indeed", nor of those who die rejecting faith: for them have we prepared a chastisement most grievous."(Qur'an 4: 17-18).
In many verses of Qur'an, God promises to forgive sins of those who believe and do good works (Qur'an 47:2, 29:7, 14:23 etc.)
In the Islamic theology, the animal sacrifices or blood are not directly linked to atonement (Qur'an 22:37: "It is not their meat nor their blood that reaches Allah. it is your piety that reaches Him..."). On the other hand, the sacrifice is done to help the poor, and in remembrance of Abraham’s willingness to sacrifice his son (according to the Muslims, Ishmael) at God's command.
Prayer and good deeds can also be atonements for sins (Qur'an 11:114). The Islamic Law, Sharia specifies the atonement of any particular sin. Depending on the sin, the atonement can range from repentance and compensation of the sin if possible, feeding the poor, freeing slaves to even stoning to death or cutting hands.
Some of the major sins are held to be legally punishable in an Islamic state (for example, murder, theft, adultery, and in some views apostasy; see sharia). Most are left to God to punish (for example, backbiting, hypocrisy, arrogance, filial disrespect, lying).
http://en.wikipedia.org/wiki/Sin#Islami ... nt_for_sin