Human is the term for the group of characters who are normal, mortal homo-sapiens that appear in Vampire Knight.
Humans have long been the regular food for vampires. Pure humans are subject to becoming Level E if bitten by a Pureblood Vampire. However, if they have any vampire relative among their ancestors, the bite won't affect them because they already have vampire blood running their veins. Hence, they will remain in human form for all other effects.
Against the menace the vampires represented towards humans, they started the Vampire Hunters Association. This group develop a long rivalry between humans and vampires, which Kaien Cross pretends to end with the foundation of Cross Academy. Most humans, however, are totally unaware of the existence of vampires and those that do find out often have their memories erased.