According to Islamic teachings that I have heard (I won't go into the 4 different theologies or Madhab of Islam) there are some things that must not be eaten unless there is no other alternative.
(taken directly from the Halal Wiki)
Pork (i.e., flesh of pig)[Qur'an 2:173]
Blood[Qur'an 2:173]
Animals slaughtered in the name of anyone but Allah. All that has been dedicated or offered in sacrifice to an idolatrous altar or saint or a person considered to be "divine"[Qur'an 2:173] [Qur'an 5:3]
Carrion (carcasses of dead animals)[Qur'an 2:173]
An animal that has been strangled, beaten (to death), killed by a fall, gored (to death), savaged by a beast of prey (except by a human)[Qur'an 5:3]
Food over which Allah's name is not pronounced (or at least not in a name other than Allah)[Qur'an 6:121]
Alcohol and other intoxicants[Qur'an 5:090]
I can positively say that this is the way it is taught, but not all are followed (alcohol for instance
) I have also been told that smoking in some sects of Islam is not allowed, but that could very well be the interpretation of the Afghanistan Sunni (Very orthodox! and in my mind misguided).
As far as Shellfish go, I know all fish are ok, and that any Muslims that I know eat all types. When I asked about Sharks, being a predator, I was told it comes from the sea so it is ok to eat. I take this to mean all things from the sea are Halal.
As far as Judaism the rules are set out more specifically.
(Again taken directly from the Kashrut Wiki "Kosher")
Only meat from particular species is permissible. Mammals that both chew their cud (ruminate) and have cloven hooves can be kosher. Animals with one characteristic but not the other (the camel, the hyrax and the hare because they have no cloven hooves, and the pig because it does not ruminate) are specifically excluded (Leviticus 11:3–8).[6][7]
Please take all this information with a grain of salt, I am most decidedly NOT a religious scholar and am only relating the beliefs as I understand them.
Cheers