What's the difference between those two? Aren't Inhumans supposed to be an evolution of the human race "forced" by the Kree? Which is almost the same as Mutants, the natural evolution of the human race...
What exactly am I missing?
They are, indeed, very similar. The main difference is very technical, based on the "origin" of their non-human genetics, and how those changes are activated.
Mutants are humans that possess a genetic variation known as the X-Gene or X-Factor, which (at least in the main continuity) was introduced into the human population by the Celestials. Biologically, they are classified as the sub-species Homo sapiens superior, as opposed to normal humans, Homo sapiens sapiens.
Mutant powers activate naturally, either at birth, or more often, at puberty. This is because the X-Gene appears on the 23rd chromosome, the sex chromosome, which mostly affects the production of hormones related to sexuality and puberty.
The Inhumans are humans that descend from a very primitive group of Homo sapiens sapiens that was genetically manipulated by the Kree to encode the potential for enhanced mutations into their genetic code. Biologically, they are classified as Homo sapiens inhumanus.
Inhuman powers would normally remain dormant for their entire lives. They must be exposed to the Terrigen Mists, a process called Terrigenesis, in order for those powers to activate. (In the MCU, for example, there are clear changes in DNA between pre- and post- Terrigenesis Inhumans.)
Despite their differences, X-Gene mutants and Terrigenesis mutants are extremely similar. The process by which their powers manifest once triggered is largely the same: the mutation produces new proteins, which in turn trigger changes to other bodily systems, which result in powers.
In particular, during the Son of M storyline, most mutants were de-powered: their X-Gene activity was suppressed and their mutant powers went away. However, exposing a former mutant to Terrigenesis restored their powers, which indicates an extremely close link, genetically, between the two subspecies.
Since all three (human, Inhuman, and mutant) are still the same species, they can freely breed with each other and produce viable, fertile offspring.
As far as their genetic mutation goes, the X-Gene appears to work like any other human gene, while the Inhuman genetic changes appear to be highly dominant, based on the known pairings:
The last bullet is particularly important in the Inhumans story line, because there are a whole lot of human/Inhuman hybrids; there are entire groups (e.g. the "Lost Tribes") of Inhumans that intermingled with human populations over the centuries, and their offspring are capable of being activated by Terrigenesis. These are commonly called the "new Inhumans", for example see this list: http://marvel.wikia.com/List_of_New_Inhumans
The primary difference between mutants and Inhumans is causal. The technology which creates the ability to have mutants in the Human population was artificially implanted in Humanity by the Celestials and has been exploited by the Kree allowing the creation of the Inhumans.
The Inhumans are merely a subset of humanity where further genetic experimentation and eugenic breeding, first by the Kree and then later by the Inhumans, has led to the ability to trigger a process of specialized abilities similar to mutant powers artificially using the Terrigen Mists. This process has happened on other worlds where the Kree have experimented, as well. This process was meant to help artificially jumpstart the evolution process again in the Kree species.
In the Marvel Universe mutant development is a genetic modification made upon the Human genome using an unknown technology by the Celestials. The modification has been made to every human, active mutant or not.
The mutant property is available to any Human being but may only be made manifest by an unknown aspect of recombination in genetic properties. This means any Human in the Marvel Universe born on Earth during the period where Humanity was recombinated with Celestial mutant markers will bear those markers and is thus capable of producing mutant offspring.
Having these markers does not guarantee your offspring will have mutant powers however. Nor is it easy for anyone to distinguish the potential for mutant development from two non-mutant parents. Whatever the technology which causes the development of the X-gene and the resulting abilities is still unable to be isolated and distinguished. Mutants only seem to be able to be easily detected once puberty and their mutant abilities are developing.
The Inhuman genetics are a modification of the Celestial adaptation to the Human genome, created by the Kree in an attempt to jumpstart their own species formerly active Celestial genetic technology which in the Kree has appeared to have reached its apex. The Kree no longer "evolve" mutant capabilities. Their Celestial technology has been rendered inactive, perhaps indicating the Kree are as evolved as they can be.
The Kree, in order to restart that Celestial technology engine for mutation began performing experiments all over the galaxy on species where the Celestials made modifications.
Their experiments yielded an ability to cause specified genetic patterns to be activated (randomly) through a process called Terregenesis. There were eugenic programs where the Kree isolated families whose genetic potential yielded potential mutations. These families were bred and their genomes mapped until the process could be activated artificially using the Terrigen mists.
Inhuman DNA still resides in many of the Humans on the Marvel Earth and they were not all living within the Great Refuge with the Inhumans. Some Inhumans refused to enter the Great Refuge, others had the ability to leave using their abilities. Once the forcefield which isolated the Inhumans from humanity was destroyed, Inhumans who could pass for Humans found their way into the Human populations further spreading their genetic material.
Mutants are natural by gen-x. Inhumans are created, mutated by technology. Practically they are the same, but genetically they are not the same. I don't know how it works, but there must be some facts in Marvel Universe. But for me this is messed up.
But mix of two "not-normal" human kind, maybe can do Xhuman or inhuman, or even human. Because everything is in us, every genes.
Apart from the Marvel Universe, genetics says grandfather genes, possibly even great-grandfather genes can be inherited. Maybe in Marvel this can be too.