A human is just a regular ole' boring-ass person of the Earthling variety. Nothing particularly special about them all whatsoever. In most situations they are the most dominant life form on the planet. On rare occasion, human begins are either conquered or superseded by another, often advanced race of beings. Some supernatural creatures moonlight as humans during a portion of their lives such as werewolves. Other creatures were once human, but became something else due to any number of unforeseen circumstances. This applies to vampires, ghosts, mummies and zombies. Other monsters, such as demons were never human at all whatsoever, but may trick others into believing that they are human. Normal human beings have no special attributes other than a tendency to eat a lot and an insatiable propensity for watching reality television.
According to the Hitchhiker's Guide to the Galaxy, humans of Earth, and everything else associated with the planet are considered "Harmless". A revised edition of the guide expanded on the sentiment slightly, upgrading humanity's threat status to "Mostly harmless".