voldemort and his followers kill many wizards and non-magic people. voldemort hears part of a prophecy saying that a specific boy might grow up and defeat him one day. he decides this boy must be harry potter, then still a baby. he finds harry's family, kills his parents, and tries to kill harry, but the spell backfires due to lily's loving sacrifice. because of his horcruxes, voldemort does not really die. he is "less than the meanest ghost" but he is still in the world. his spirit runs away and hides. even with voldemort gone, most wizards are still so afraid of him that they do not want to say his name. they call voldemort "you-know-who" or "he who must not be named".