Display of cooperation at cost to self.
Evolutionarily these costs and benefits are measured in terms of Darwinian fitness and it is not so much that altruism necessarily lowers one's own fitness (though it can) but instead that it has the effect of raising the fitness of one's potential to higher than one's own.
There are at least two basic explanations for why animals would be altruistic. The first involves what is known as inclusive fitness and/or kin selection while the second involves what is known as (or reciprocal altruism). See, though, also .
In the following video Richard Dawkins considers altruism from the perspective of both The Selfish Gene and human societies.