Altruism refers to both a practice or habit (in the view of many, a virtue) as well as a philosophical doctrine.
Altruism the practice is, briefly, charity, or being particularly helpful to other people with little view to being rewarded for one's efforts.
Altruism the doctrine is the view that one's actions ought, generally, to further the interests or good of other people, if necessary to the exclusion of one's own interests. The word was coined by Auguste Compte, the French founder of positivism. Altruism is distinguished from ethical egoism, according to which one's actions ought to further one's own interests.
Like all broad philosophical isms, what altruism entails is not at all clear. In developing a precise version of altruism, one would have to decide exactly what "interests" or "goods" are, explain what "furthering" these interests includes and excludes, and distinguish between one's own and others' interests (this is not always clear).
According to psychological egoism, while one can be outwardly altruistic in the practical sense, one cannot have altruistic motivations. That is, while one might very well spend one's life helping others, one's motive for doing so is always the furthering of one's own interests (a purported altruist might derive great pleasure, for example, from helping others, and that pleasure is the motive for the helping).