Marvel is planning an Inhumans film for 2018, and may-or-may-not be revealing some Inhumans on the Agents of SHIELD TV series, but whenever they're coming to the Marvel Cinematic Universe, many are wondering: Who or what are the Inhumans?
First the info that is actually relevant - then all of the comic booky stuff that you don't need to know (but I'll throw in anyway).
They look normal until Terrigenesis
The most basic fact about the Inhumans is that they are born looking completely human. Sure, the purebred Inhumans might be a big stronger and longer lived than your average human (with the trade-off of a weakened immune system), but wherever they come from, most Inhumans look perfectly normal - until Terrigenesis.
Traditionally, the process was a rite of passage in which young Inhumans deemed worthy enough would be exposed to Terrigen Mist (originating from Terrigen crystals) which would send them into a cocoon-like state, transforming them at a genetic level, until they emerge as something greater. Terrigen Mist is fatal to most normal humans, but a small-but-significant portion of human population have Inhuman genes (more on them in a bit), so they too will undergo the change.
Terrigenesis-transformed Inhumans can have any sort of power, from something as innocuous as the ability to filter air to something as intense as the ability to destroy anyone with a look. Physically they can look completely human, or they could be a tree-man, or a walking piece of charcoal, or - in at least one case - a wall-sized door.
The Royal Family rules in Attilan
The first and greatest Inhuman city is Attilan, also known as the Great Refuge. Within the city's walls, Inhumans are safe from the ravages of humanity, protected from man-made pollutants, and have their needs catered to by technology thousands of years more advanced than anything else on Earth.
The city is ruled by a governing body called the Genetics Council, but presided over by a king or queen, usually the most powerful among the Inhumans at any given time. For over a century, one Royal Family has run Attilan, led primarily by Black Bolt, the mute but deadly king. Black Bolt is incredibly strong, durable and has various energy-related powers, but his greatest asset (and curse) is his voice. With the merest whisper, he can level a building - so he has vowed never to speak. Interpreting his silent orders is his queen, the follicley powered Medusa (who can do anything with her steel-like prehensile hair). The Royal Family also includes the bull-like Gorgon, the aquatic Triton, the elemental Crystal and (until recently) the strategist and martial artist Karnak.
Everyone will love Lockjaw
Also part of the Royal Family, is their pet: Lockjaw. He's going to be the Rocket Raccoon or Baymax of any Inhumans franchise.
Apparently just a dog that was "accidentally" exposed to Terrigen (but was not an actual Inhuman person, as some have suggested). Lockjaw is nearly as strong as the Hulk and has various energy-based abilities, but his primary duty is the Royal Family's teleporter. Lockjaw can take anyone to any point in the world or other dimensions (which makes taking him for a "walk" a pain in the butt). He is, however, the nicest and most loyal dog that had his own team of Avengers.
Inhumans are older than human history
Over 25 millennia ago, Kree aliens experimented on the primitive populations of hundreds of worlds as part of their Metagenesis plan to develop superhuman mutations. Realizing the creations might end up more powerful than their creators, the Kree ordered all experiments terminated, but as such things always happen, some groups, including the Inhumans of Earth, escaped the culling.
Following their genetic programming, the Inhumans discovered Terrigen, and overtime began to celebrate their genetic diversity, but not everyone liked living in a superhuman melting pot. They hid from the growing threat of humans who outnumbered the Inhumans a million-to-one. Various factions broke off from Attilan to form their own separate kingdoms, and overtime there would be several "lost tribes" hidden throughout the world, including the Birdmen of Aerie, Mayapan "gods" (impersonating the actual Mayan gods of the MU) and most recently Lash, survivor of the dead city of Orollan. He's taken it upon himself to decide who deserves Terrigenesis, and who deserves to die.
Because of all of the lost Inhumans out there, thousands of people in the Marvel Universe are unknowingly descended from Inhumans, meaning they too transform during Terrigenesis. Recently Black Bolt unleashed a cloud of Terrigen Mist, and as it circles the globe it is transforming thousands of seeming humans into actual Inhumans. These newbies are called "Nuhumans" or "NuHumans."
There are Universal Inhumans (and royal polygamy)
Recently, the Inhuman-like cultures of four alien races, all of whom escaped the Kree Metagenesis culling, brought their populations to Earth. The queens of each civilization married Black Bolt as part of an ancient intergalactic prophecy. As a result, Black Bolt has one wife from Earth (Medusa), one blue-skinned Centaurian wife (Oola Udonta), one blob-like Dire Wraith (Avoe), one horse-like Kymellian (Onomi Whitemane), and one hairy reptilian Badoon (Aladi Ko Eke). They generally agree Medusa is the first among them, but don't think this intergalactic polygamy isn't awkward.
This also means that, aside from the few thousand Attilan Inhumans, hundreds or thousands of Inhumans in lost cities, and the unknown thousands of "Nuhumans," there are several thousand Universal Inhumans from these four alien worlds.
Additionally, Black Bolt is not one to sit on his laurels. Just as space has influenced his kingdom, he has influenced space. During his rule, Black Bolt has warred with the Skrull Empire, conquered the Kree who created his people, defeated the Shi'ar Empire and generally made sure his presence was felt everywhere he went. He has currently abdicated all of his various leadership roles and is focusing on saving all of reality from impending doom.
The Inhumans have slaves
Every society has a dark side, and the Inhumans are no exception. Aside from their general predisposition towards genetic superiority and their own caste-like system determining worth, they also have an entire slave population.
Roughly 4,000 years ago, the Inhumans found themselves in need of a labor force, so they created one. The Alpha Primatives were artificially created and grown to be powerful, docile, genderless and with the most rudimentary of intellects. They served peacefully for most of the time since, though occasionally sympathetic Inhumans would try to free the Alphas. Unfortunately, their genetic programming usually meant they would eventually return to their role as laborers, whether they were forced to or not.
A few Alpha Primatives have moved beyond this, with some gaining gender dimorphism through various means resulting in the creation of more sapient offspring, and others becoming godlike through their exposure to Terrigen Mists. Although the Inhumans have largely stopped producing more Alpha Primatives, they do have access to Xerogen crystals which do not affect Inhumans, but can be used to turn regular humans into Alpha Primatives. There's always room for more workers.
Well, there ya go, everything you ever didn't want to know about Inhumans because you thought it was unnecessary to ask. Anything else?