Zombies are the animated dead. In their usual depictions in movies and TV shows, they are shambling, disintegrating, and mindless corpses who seek to kill the living so they can eat their brains.
Sometimes, they are controlled by magic so they appear to be intelligent but are really robots of dead flesh.
The traditional zombie is most often a mindless threat, part of a massive hoard of stupidity and appetite which moves toward its victims who must find a safe fortress to fight against them.
The creation of the zombie is as often a scientific one -- a pandemic virus or a scientific genetic experiment gone wrong as it is a supernatural one caused by curses, demons, or evil magic doers.
The zombie also appears under a number of other guises including the failed/feral vampires in Charlaine Harris' Sookie Stackhouse novels and Davidson's Queen Betsy novels.
Other writers like Marjorie Liu in her "Hunter Kiss" urban fantasies used demons to infest the dead.
In most novels, the zombie is a threat, but it is rarely the main threat since it lacks the sentience that a good bad guy needs in a novel. This lack of sentience doesn't matter in a movie, but it weakens a novel which is more intimate.
An effective bad guy in a novel reacts to the main characters, he has snappy dialogue, and he threatens his minions and everyone else. A zombie does none of those things.
Some writers have turned the zombie myth around by making the main character a zombie who is both sentient and blood-thirsty. For example, Mark Henry in his Amanda Feral series had a zombie chick-lit heroine who was smart, witty, and a cannibal. Black humor and horror are always included.
The zombie as a romantic partner in a romance has also happened which I find gross. Your tastes may vary.
NOTE: Welcome back, iZombie, the only zombie show I’m a fan of.