Simulations are "sim" games so l4d, dead rising, ect are infact "sims" anything that simulates a zombie apocalypse
it simulates zombies trying to kill you.
SCZ simulates survivors in certain buildings, survivors that wish to follow you, bit people turn into zombies, you have the ability to combine certain things, use special abilities like putting a turret infront of a door.
I SHEWT ZAWMBAYS AND CHANGE GUNS
EDIT: SCZ also makes your screen blurry and makes you walk slower if you get tired or hungry.