Inhumans are a set of characters that first appeared in Marvel comics, they were later depicted in the TV science fiction series Agents of Shield and will also feature in their own TV series to be released in 2017 in IMAX before premiering on TV.
They bear many similarities to the idea of the mutants from the X-Men series, however their origins are primarily alien and technological in origin, hence whilst all mutants are or were humans, not all inhumans necessarily consider themselves as such.
What are Inhumans
Inhumans are the result of an effort by what are known as "Kree" to create an army on Earth using their genetic material with that of humans. Thus the term "Inhumans".