Inhumans were normal humans given powers when Mr. Sinister detonated his Terrigenesis Bomb in Central Park. Everyone who was in or around Central Park at the time was either given great powers or horribly mutated. Since the bombing, Central Park was closed down due to potential Terrigenesis residue remaining in the area.
In the New Avengers Unleashed story arc, Mutant Mayhem, the Avengers and the Alpha Flight tracked down mutate terrorist Mr. Sinister, and his team of Marauders. They killed Mr. Sinister, but not before he revealed he wasn't a genetic mutate but an "Inhuman". He created a special super power serum known as Terrigenesis, and he injected it into himself to give himself superhuman abilities. Each person exposed to Terrigenesis has different mutations. The serum gave Sinister night vision, superhuman agility, and razor sharp fangs, but it also left him with albino skin and red glowing eyes. Right before being killed, Sinister detonated a Terrigenesis bomb which he had planted beneath Central Park. Everyone in or around Central Park was transformed into an Inhuman like Sinister, each with different mutations and powers.
Inhumans & Humans
After the Terrigenesis bombing, the government decided to let the Inhumans live free lives much like they did before the bomb went off. This caused a lot of controversy. However, Inhumans were still humans, just with super-powers, so they should still be treated like normal people. After all, plenty of superheroes are basically the same way. An Inhuman couple, Blake Boltagon and Madelyne Amaquelin, sued their landlord, Mr. Forbush, for his hostility toward their kind. Their landlord was fined and forced to let the two stay in his apartment building, but the two left anyway, not wishing to live under the roof of an Inhuman-hating man. The two moved into Attilan Towers, an apartment complex that actually welcomed Inhumans. Inhumans soon became very popular, and even more media attention was given to them when Madelyne Amaquelin's sister, Crystal Amaquelin dumped her famous boyfriend, Johnny Storm, a member of the Fantastic Four because he only wanted to date her because dating an Inhuman would be good for his public image.