Jiang Shi are, simply put, chi vampires/zombies. they function kind of like a type of avatar: a hunger spirit is summoned, it inhabits a persons body, and takes over. the real person is trapped inside, unable to die or escape. the JS spirit isn't bound to the human soul, but neither can escape from the body, except in death.
in appearance, they look much like a corpse. (depending on how long ago they were created, they may be at varioius stages of decay.) at will, the hunger spirit can change the human body, kind of like a wesen (pronounced "vesen") from Grimm into a horrible humanoid-lizard like form. as we saw in Beginnings, spirits are able to change a human like this.
there are two "breeds" of Jiang Shi: the first breed is the breed we have already seen in the story, later to be known as Screamers. Their screams have the power to knock someone out and drain their chi, as we've seen, and they are particularly vulnerable to extremes in temperature, as they don't have any extra armor or anything. the new breed we will see after fighting with Xio will be called Supers (maybe... if you have another name suggestion, i'm totally open). They don't have the ability to scream as we've seen, but when transformed, they have a thick, crustacean-like armor plating covering more or less their entire body. this armor has a resistance to extremes in temperature, so they are much harder to defeat. however, if they are exposed to heat for too long, the armor will eventually begin to melt, and once their insides are exposed, they burn up very quickly, like flash paper. (screamers do this as well, but they don't have any armor to get through.) Extreme cold takes much longer to have an effect on them, but it can crack their armor, and then freeze their insides. the problem with this, however, is that once they thaw out, they are back to normal, and can feed on chi to regenerate their armor.