The term speciesism relates to a moral superiority that puts human interests above some or all nonhuman animals or the assigning of a hierarchy to animals in relation to their value.
Speciesism is an ideology—a belief system— that legitimates and inspires animal oppression by fusing moral significance with (a) membership in the human species and (b) human cognition. Speciesism is used to exclude nonhumans from the moral community because they do not belong to the species Homo sapiens and because (most of them) do not possess our peculiar form of cognition. This ideology thus creates a hierarchy of moral worth, with human animals at the apex, followed by all other animals, who are measured in terms of their cognitive nearness to or distance from humans.
Prior to our understanding of the physiological similarities and common evolutionary origins between species, the devaluing of animal interests could easily be supported by reference to common sense about superficial differences (such as inability to “speak” to humans), to religious texts, or to intellectual ideas made popular for their usefulness (such as those of Descartes, who saw animals as machines that did not feel pain).
Today there exists a pervasive cultural socialisation that ensures that many take our oppressive relationship to other species as “natural”. It is this, combined with vested corporate and state interests, that maintains exploitation on an unimaginable scale.
These barriers to equality remain to the detriment of all life on Earth. For the abuse of power through hierarchical order manifests in many interacting problems. Just as unequal power relations between nations, genders, races, ethnicities, classes, and sexualities have resulted in the historical oppression of certain groups, so unequal power relations (as a result of different evolutionary adaptations) have resulted in the historical oppression of all other species by one.
“Whenever you see a bird in a cage, fish in a tank, or nonhuman mammal on a chain, you’re seeing speciesism. If you believe that a bee or frog has less right to life and liberty than a chimpanzee or human, or you consider humans superior to other animals, you subscribe to speciesism. If you visit aquaprisons and zoos, attend circuses that include “animal acts,” wear nonhuman skin or hair, or eat flesh, eggs, or cow-milk products, you practice speciesism. If you campaign for more-“humane” slaughter of chickens or less-cruel confinement of pigs, you perpetuate speciesism.” 
For more on the similarities between speciesism and other forms of discrimination, and how we can acknowledge these similarities in our activism, see our booklet on coalition building: