Selfishness

Selfishness is, at base, the concept and/or practice of concern with one's own interests in some sort of priority to the interests of others; it is often used to refer to a self-interest that comes in a particular form, or above a certain level.

Types of Selfishness
Selfishness is usually associated with a deliberate act. For example, a selfish person deliberately focuses on their own agenda, rather than that of others. The act of being selfish can also be unconscious or accidental. This type of selfishness is no less valid and may actually be more destructive to an overall society, due to its less obvious nature. Examples of unconscious selfishness may be so small as to not be noticed, but often occur in large quantities.

As it is likely that most people have performed a selfish act without knowing, it is probably more correct to say that un-selfishness is a deliberate act, rather than selfishness, which tends to occur naturally.

Typical acts of unconscious selfishness may be to obstruct public pathways (i.e. to stand in the way when there is no need to). Also the lack of correct roadcraft, which may hinder other roadusers. Littering may be considered selfish, but cannot realistically be considered unconscious.

Selfishness regarded as good, or a healthy thing
There are some non-religious philosophies that hold a positive view of selfishness, usually on the basis that it isn't what the common usage refers to, and that the identification of 'promotion of the self' with 'evil' is an unhealthy practice that actually devalues some good qualities such as productivity or the taking of personal responsibility. One view is that since one needs to act in a mainly self-interested way in order to advance in life doing so should not be regarded as wrong, or labelled as harmful or inappropriate.

Similarly, an individual might ask himself why he ought to choose to act unselfishly anyway if he has no guarantee in advance that others in the world will not act selfishly. One will tend to act selfishly for one's own self-protection, in a world where one mainly encounters others doing the same.

Group selfishness (as compared to individual selfishness)
Selfishness usually refers to the self - that is, to the individual. However, in common speech, a group of people can be accused of "selfishness" too, in the sense that members of that group are not concerned with the welfare of anyone outside their group but are only inward-looking: concentrating on the needs of the group. This may in some circumstances be characterized as indirect self-interest.

General
Based on the theory of the iterated prisoner's dilemma, evolutionary biologists and game-theorists come to the conclusion that selfishness is - besides cooperation among relatives and genetically programmed behaviour - the basis for cooperation among individuals of the same or different species.

While some would characterize Selfishness as the opposite of Altruism, a more indepth understanding of the nature of the two ideas (see Dawkin's Selfish Gene and the Prisoner's dilemma) and how they tend to overlap or follow directly from one another, leads others to consider it to be a false dichotomy based on artificially strict definitions.