Salvation typically refers to the process of being saved from an eternal hell, though the details of the concept and the requirements for salvation depend on the religious context in which the term is used. Not all religions include a hell to be rescued from, a heaven to be rescued to or even an eternal soul which requires rescuing.
Religious views on Salvation
The Roman Catholic church defines two sorts of sins; mortal and venial. The process of salvation relies on the individual's acceptance of God's grace and is sanctified by good works. Any individual who dies with an unforgiven mortal sin (which range from murder to missing mass or other sacraments without a good reason) will spend eternity in hell. Any individual who dies with an unforgiven venial sin (white lies and other minor sins) will go to a temporary hell (purgatory) until those sins have been paid for. Catholics habitually confess their sins to priests who forgive them and prescribe some sort of penance.
The Roman Catholic Church has also offered indulgences, though these are less popular today than in the past. By performing a particularly good task one could be guaranteed salvation. Indulgences were also used to raise money as one could pay to have loved ones released from purgatory, or even pay in advance to have one's own time in purgatory reduced.
Protestantism covers a diverse group with varying doctrines on all issues of Christianity, including salvation. Typically, Protestants believe in salvation by the faithful acceptance of the death and resurrection of Jesus (as an atonement for all sin). Good works are typically considered to be a testimony that this conversion from "sinful" to "saved" (popularly referred to as being "born again") has taken place and do not, on their own, guarantee salvation.
Some believe that salvation is predestined to a select few, others believe salvation is available to anyone. Rituals like baptism are requirements for salvation in some denominations while others hold baptism as an optional rite.
Jews reject the idea of original sin and with it the idea that salvation is required. The concept of an afterlife exists within Rabbinic Judaism and is available to all Jews as well as righteous non-jews (those who adhere to the Noahide laws). While the specifics about an afterlife vary among Orthodox, Conservative and Reform Jews, the Christian ideas of salvation, heaven and hell don't have clear analogies in Judaism.
Muslims believe that each person's deeds, beginning at puberty, are recorded and that on Judgment day those deeds will be considered, by God, in order to determine that individual's fate. Faith and God's mercy are also considered and Hell is temporary for those who had faith.
Hindusim's concept of salvation is a form of enlightenment or personal realization which breaks the cycle of death and rebirth. The final, ultimate "truth" of the nature of each individual is different.
Buddhists seek an enlightenment (Nirvana), similar to Hinduism, by following the Eightfold path which includes moral behavior and meditation. The concepts of heaven, hell and even a supreme being are foreign to Buddhism and the concept of Nirvana represents a personal achievement which ends disease, ignorance and the cycle of death and rebirth.
Atheism and Salvation
Not all atheists reject supernatural claims and many Buddhists, for example, are atheists as they do not believe in the existence of a god. Generally, those who adopt personal labels like "atheist", "secular", "humanist" are identifying their acceptance of scientific naturalism, rationalism and/or materialism. These individuals would reject the notion of souls, gods, heaven, hell, original sin and salvation as assertions which lack sufficient evidence to justify belief.