Premise
In Dead Space, you take on the role of Isaac Clarke, a system engineer for C.E.C, part of a team onboard a engineering? ship called the
U.S.S. Kellion. The ship is responding to a distress signal from a large planet cracking vessel known as the
U.S.G. Ishimura. The Kellion crashes in Ishimura's landing bay, to find the ship completely deserted. Isaac is asked to get a damage report from behind a security booth, when the lights dim, and you get your first glimpse of the enemy.
The entire crew are now horribly mutated monsters called Necromorphs. The only way to kill them is by dismemberment.
If you haven't learned by now, Dead Space is a Survival-Horror game. And I must say, the game is quite horrific.
Enemies
Each enemy can only be killed effectively via dismemberment. Most enemies also have special and gory animated Death Scenes.
Slashers are the most common Necromorph form encountered in the game. Their main weapons are the two blade arms coming out of their shoulders, with two pairs of grabbing arms coming out of the abdomen. They attack through pure melee combat (with the exception of the Female slasher, which can also shoot acidic spheres a small distance.), and when up close, like to grab Isaac and bite into his neck.
This ugly sonofabitch is a Slasher.
Brutes are a combination of multiple hosts fused together into one large being, although it's impossible to tell exactly how many bodies have been reanimated to create these monsters. They are large, hardy, and like all Necromorphs, extremely violent.
Dividers are extremely tall, thin Necromorphs that are fairly strong and hardy. Once slain, the head, arms and legs separate and become independent creatures. They are very fast and will damage and attempt to strangle you, although they are much weaker than the original Divider.

The list goes on and on with Leapers, Exploders, Pregnants, Lurkers, the Leviathian, the Hunter, Swarmers, Twitchers, Wheezers, and perhaps most disturbing, the Guardians.
Guardians are the result of the Necromorph infection anchoring a torso to flesh growing on the walls. One can usually tell when one is nearby when moaning, groaning, or screaming is heard. However, some have been observed as being silent until engaged in combat. Unlike many Necromorphs the
infected human seems to retain consciousness, hence the constant moans and wails of pain, rather than simply acting as a lifeless host. The circumstances by which this occurs have not been made clear.

If you want to know a tad more, check out the Wikia at:
http://deadspace.wikia.com/wiki/Main_PageThe game is not for the squeamish or chicken.