Scientifically, humans are a species- and the breadth of the species is all creatures which, if they have a child, that child is fertile. A horse and a donkey can breed, but the mule that results is sterile- those are different species.
If you’re looking for a behavioral definition, that’s harder, since a lot of behavior patterns we thought are unique are now being seen elsewhere. But written language is pretty unique to humans.
“What makes us human?” is a really deep question that people have struggled with for our entire existence. The human genome has been mapped, so you could define humans in terms of our DNA and genes, or you could define it in terms of our ability to create complex social structures, works of art and the like.
Scientifically, I think you’ve got to define us in terms of our genes. I think what makes us human, though, is our capacity for abstract thought, which no other animal (as far as I know…) is capable of.