Human

Humans are mortal beings characterized by their inability to practice magical or supernatural powers. As the human race repletes the majority of the Earth’s population, they are commonly noted for their desire to influence and understand the environment by seeking to explain and manipulate phenomena through mythology, philosophy, religion, and science.

Vampires
Only a handful of humans are aware of the existence of literal vampires. Among the greater masses of the 21st Century, vampires are regarded as superstition, fiction, and myth. Humans can be turned into a vampire, with vampire blood and then death, although it needs to happen within 24 hours. Some humans can be completely unaware of what's happening to them during their transition into vampirism, and will feel confused and very hungry, much like Caroline Forbes when she first turned.

It appears that humans are more aware of vampires in New Orleans, than anywhere else.

Werewolves
Not very many humans are completely aware of werewolves, mainly due to their near extinction, making them an endangered species. Humans can also give birth to werewolves. Werewolves are technically still human until they trigger the curse (Tyler Lockwood being compelled by Damon Salvatore and Eve not being able to heal at a rapid state, being proof). They can pretty much grow up living a human life unaware of their heritage, such as Hayley Marshall and Richard Lockwood.

Witches
Some humans might not be completely aware of witches, but they are most likely aware in New Orleans, and sometimes used in human witchcraft practices or sub-culture classes. Back in the Salem witch trials, humans were held responsible for the burning of the witches, especially Emily Bennett. Despite the horrible history between the two species, it appears that witches can generally get along with humans, possibly mainly because they are bound by nature. Humans can give birth to witches, and can also lose their powers, becoming human.

Witches (much like werewolves) can also be completely unaware of their heritage and will most likely confuse them when their abilities start to break out, much like Bonnie Bennett.

Doppelgängers
Only a handful of humans are aware of their existence. Humans can give birth to a doppelganger and a human can become one, if they took the immortality spell, and distributing the balance of nature. Despite still being human, a doppelganger is still considered supernatural.

Ghosts and Mediums
Only mediums and witches can communicate a ghost. The only way a human can make contact with a ghost is if the veil to the other side breaks down, meaning ghost domination. Mediums are still human, but with special abilities. They are humans who died (non-supernaturally) and have been resurrected by a witch, possibly leaving a "door open" for contact with ghosts.

But since the other side is destroyed it is unknown if mediums can still communicate with ghosts.

Supernatural Hunters
Humans are little aware of supernatural hunters and what they stand for. A human can become one, if they can notice the hunter's mark on another hunter, then they become awakened after the other hunter dies. With the enhanced strength and agility, supernatural hunters will have strong urges to kill vampires, even when they don't want to, or have never had a problem with vampires in the first place.

Trivia

 * Humans can be turned into vampires.
 * Humans can give birth to doppelgängers, werewolves, or witches.
 * Humans can become mediums or supernatural hunter.
 * The human race is the oldest species known to man, alongside witches and possibly werewolves.
 * Humans generally have a short life-span when compared to other sentient species. Vampires and hybrids are ageless, witches can slow down their aging, and ghosts can keep on existing and influencing the world around them if they find a way.
 * Werewolves are the only other species that has the same life-span as a human.
 * Rebekah and Elena miss being human. Stefan missed being human.
 * Humans can possess psychic abilities such as Jeremy's mediumship or Bonnie's visions, but this however can be considered her witch powers breaking out.
 * Jeremy got his powers after he died.
 * The human race is the most fragile species in the series as they can die from many weaknesses such as heart extraction, natural causes, illness, etc.