Altruism is acting out of concern for another’s well-being. Often, people behave altruistically when they see others in desperate circumstances and feel empathy and a desire to help.
"Reciprocal altruism" is a term used by evolutionary biologists and psychologists to characterize the decision to help with an expectation that one will receive some benefit or payoff to oneself. Even when people don’t expect recognition or reward for a good deed, however, they often feel energized and happy afterward, a sensation sometimes called the "helper's high."
Cooperative behavior allowed our ancestors to survive under harsh conditions, and it still serves a purpose in a highly complex society. Humans aren’t the only animals who behave altruistically, though: Many species benefit when individual organisms disregard personal costs and act in service of the larger group.