Humans are the basic species in the Twilight series, from which vampires, shape-shifters, and werewolves originate. Lacking vampiric enhancements, they are considered prey by most vampires, who need their blood for sustenance.
Human beings who discover the existence of vampires are considered a threat, and a Volturi-enforced law has been implemented dictating that said humans either be killed or changed into vampires, causing Jane, Alec, Demetri and Felix to be sent to Forks in Eclipse. It is also illegal to create immortal children, that is to turn human children into vampires, because they always lack concentration and self-control, and are therefore un-trainable and dangerous.
A human who is to be changed into a vampire needs just to be bitten once, to allow venom to enter their bloodstream. If blood is consumed by the vampire during this time, it is highly unlikely that he will resist the temptation to kill the human before completion of the transformation. The pain accompanying the transformation is described as the worst possible by all those transformed into vampires, and in Breaking Dawn, it is described as a burning feeling by Bella. A human, however, can also be changed into a werewolf by being bitten by one, and it remains unknown whether the conversion is painless.
According to Aro, the human race has weapons that could destroy vampires, but little chance to use them, given the latters' strength and speed, and the fact that their existence is considered mythical. The situation, however, worries him.