Superhuman is the hardest mode in Resistance: Fall of Man, Resistance 2 and Resistance 3. Superhuman is unlocked after beating Hard Mode or Difficult Mode in Resistance: Fall of Man or Resistance 2 and Resistance 3, respectively.
Completing Superhuman on Resistance: Fall of Man and Resistance 2 gives you prizes which are the Black Ops suit for online multiplayer and also the Skull head (which you also need all skill points for) in RFoM, and the Trophy OMGWTFBBQ in Resistance 2.
In Superhuman difficulty for Resistance: Fall of Man, enemies will appear in various places throughout the game where they were not there in those places in lower difficulty. Enemies will also deal more damage and health is lost more easily.
In Superhuman difficulty on Resistance 2, all Standard Hybrids have been replaced by Advanced Hybrids wielding Bullseye Mark II. Bosses also have increased health. Your character is also given less health and enemies deal more damage. Weapons still hold the same amount of ammo no matter the difficulty.
In Superhuman difficulty for Resistance 3, enemies will deal more damage and health is lost more easily. Completing Superhuman without cheats will earn you the Brutal trophy.