A human being was famously defined in antiquity as a "featherless biped". Biologically, humans, or homids, are a group of several closely related species of ape-like animals, distinguished by tool as language use as well as genetic markers. Modern humanity belong to the species of homo sapiens (wise hominids), a species that has been in existence for approximately 250,000 years ago.
As to what happens when we die: First, death is normally now defined as the cessation of electrical activity in the brain, or "brain death." Although medical technology can keep the body functioning after brain death, in so far as our brains or consciousness are what define us as human, once the brain ceases to function we no longer have the essential character of a living person.
Various philosophers and religious believers argue that some non-physical aspect of a person may persist after death, while others argue that the death of the body is an absolute end to a person.