Altruism is an expression of concern for the welfare of others without any obvious benefit or motivation on the part of the individual expressing the concern. It is actually quite difficult to find examples of true altruism in the psychological sense, as many acts which are perceived as altruistic actually carry psychological rewards. In humans, helping others generally results in a feeling of pleasure or satisfaction, suggesting that humans have actually developed a biological imperative which pushes them to behave in an altruistic manner by providing an emotional reward.
When people talk about altruism, however, they don't usually mean altruism in the biological or psychological sense. They refer to an act of kindness or expression of concern which does not carry a material benefit. For example, someone who donates money to a charity might be considered altruistic, because he or she does not benefit directly from the act. Conversely, someone who builds homes for the homeless for school credit is certainly performing a charitable act, but not an altruistic one.
Many cultures view altruism as a positive personality trait, and altruistic acts are in fact highly valued. Many religions including Christianity, Buddhism, Judaism, and Islam promote altruism among their followers, stressing the idea that people are not righteous if they cannot express unselfish concern for others. The ability to help others, even at personal cost, is considered an important part of the expression of religious faith for many people.
In biology, you may hear people discuss altruism in the sense of an act which may harm an individual, but advance the species or social group. Biologists also talk about reciprocal altruism, which could better be termed “cooperation,” since it involves helping another organism with the understanding that the other creature will return the favor. “I'll scratch your back if you scratch mine,” in other words. Organisms which fail to cooperate may be punished, enforcing mutually beneficial behavior by showing that lack of cooperation has consequences.
Ethical theorists also talk about altruism. In ethics, altruism is often described as an imperative, with some ethicists arguing that in order for a society to survive, it must be altruistic in nature. For example, some ethicists support the use of taxation to raise funds for the purpose of providing health care to all citizens, something which benefits low-income members of society more than high-income members of society while also supporting the health of the society as a whole.