'Undead' is a large umbrella term used to define a group of beings that are 'alive', but 'dead' at the same time. This includes things such as golems, ghosts, vampires, and zombies (but not Voodoo Zombies. Undead are extremely rare in the city, and most people outright refuse to believe they even exist. As such, most people will never come across a member of the undead, unless of course they are being preyed upon.
The zombies mentioned here are being that were actually dead, but managed to physically come back to life some way of the other. There are a wide range of zombies, to the mindless flesh eating variety to the 'intelligent, but just happens to be made of body parts' Frankenstein's Monster. Zombies can only be created by either paranormal or magical means (like Necromancy or SCIENCE) so they are incredibly rare. Zombies in this universe cannot turn living creatures into zombies by biting them, but however that's really gross and if you are bit by a zombie you need to get that cleaned out at a hospital!
Vampires are the most common member of the undead, and are found pretty much everywhere. Vampires have their own complex society and rules that most conform too, but once in a while a few will scratch it out on their own. Vampires must feed on humanoid blood to stay 'alive', though they can also feed on animal blood, as long as it is fresh'. Vampires are also able to drink other things, such as alcohol for enjoyment, but they get no nutritional value from it.
Vampires come about from a regular humanoid (mutant or human) drinking blood from a living vampire. This usually happens after a vampire drains a victim, and they offer their victim a choice to die or become a vampire. If the victim chooses to become a vampire, they drink blood from their 'sire' (the vampire that converts the humanoid to a vampire), and that person then becomes a vampire themself.
Vampires have many abilities up their sleeves, such as the power of hypnotism, shapeshifting, enhanced strength/reflexes/senses, and immortality. However, they are prone to 'classic' vampire weaknesses, such as garlic, salt, crucifixes, holy water, and sunlight.
Vampires are extremely long lived, with the oldest nearing nearly 1000 years in age. However, due to the fact that they prey on people, they are relentlessly hunted, and so vampires exist in a delicate predator/prey balance. Due to this fact, new vampires are only created as they are needed, and those that create new vampires without permission are usually punished (usually with death)
Ghost tend to come about due to people dying either through very violent means or someone with a high passion about completing a task before they can move on. Ghosts of course can come in several different types such as repeating ghosts (ghosts with no intelligence and just repeat a task over and over), 'Normal' ghosts, and Poltergeists. Most ghosts exhibit some forms of higher intelligence but rarely can they be completely coherent, due to the trauma of death. Most ghosts can have limited conversations with people, though usually it's tied to their last thoughts or actions. All ghosts are driven by a singular goal that keeps them from crossing over, and that can range from telling someone goodbye to getting revenge on their murderer. So thus ghosts tend to be extremely obsessive and can become violent if forced to stray from said obsession.
Usually ghosts can only be seen or heard clearly by those sensitive to the paranormal, but the average public can see and hear ghosts to some degree if they are 'strong' enough.
Golems are in a nutshell, inanimate creatures that are given life. This can be anything from sewing together dead matter to create a new creature, to things such as giving life to a porcelain doll. Golems are very very rare, and such not much is quite understood about them.