@Nullo The modern world is not built on selfishness. At most, it is built on self-interest. If nothing else, the good parts of the modern world do not seem to have been built on selfishness.
@ddude1116 It is not pointless. It is the crux of an ethical debate that goes back to the time before Plato. Nor is this debate merely an artifact of academic philosophy. It is present in everyday life quite independently of theoretical concerns, such as in the mistaken view that all acts are either selfish or altruistic. Failure to understand the difference can be found lurking behind many political debates, and the mistake infects the left and the right in equal measure.
Perhaps a simple example would help. Consider two people, Sam and Phil, who come across a ten dollar bill lying on a sidewalk. The bill is not owned by anyone, so it is up for grabs to whoever gets to it first. Sam and Phil are equidistant from the money. Here are some possible options:
1. Sam decides to let Phil have the money and does not race to get it.
2. Sam decides to race Phil to the money in the hopes of acquiring it for himself.
3. Sam decides to shoot Phil in the kneecap to eliminate competition for the money.
The first option could be called altruistic, though the circumstances surrounding and reasons for Sam’s decision could make it otherwise (perhaps, for instance, Sam decides it is beneath his dignity to race for such a small amount; the situation might change were the sum of money at stake larger). The second option would be called self-interested, for Sam decides to make an attempt to improve his own situation rather than just letting Phil have the money. The third option would be called selfish, for Sam has disregarded the interests of Phil in order to improve his own situation. He has not respected that there are limits to what we may do in pursuing our self-interest, and thus he has acted selfishly. This is a qualitative difference that cannot be cashed out merely through talk about the presence or absence of a hyphen.