I know the difference between poison and venom AND poisonous and venomous. But I just want to know if an ingested venom would do harm to humans and if an injected poison would do harm to humans. I really think that poison is both poisonous and venomous AND venom is both venomous and poisonous.
I think you are both confused about terminology, and asking a very broad question. An organism that produces venom is venomous. Poison is not venomous, the black mamba is venomous. Venom is not venomous.
According to the Wikipedia article on venom,
Venom is a form of toxin secreted by an animal for the purpose of causing harm to another.
A toxin is a poisonous substance produced by a living organism. The article goes on:
Venom is injected into victims by means of a bite, sting or other sharp body feature, which differentiates it from poison (that is absorbed, consumed or inhaled).
However, a toxin is a poison, and poisons are defined (by wiki) as
substances which cause disturbances to organisms, usually by chemical reaction or other activity on the molecular scale, when a sufficient quantity is absorbed by an organism. (emphasis mine)
So, the definitions are kind of circular.
From what I understand of your question, you are wondering whether a venom would still be dangerous if it were, for example, ingested rather than injected, as well as whether a poison would still be dangerous if injected rather than ingested (or inhaled, or whatever), and this is where the "Too Broad" category gets applied. There are thousands if not millions of different types of toxins in the world, ranging from full proteins to peptide-based to small organic molecules to completely different structures, such as lipopolysaccharide (LPS, the component of Gram-negative bacteria that causes septic shock). Some may not survive the digestive tract, others might. Some can act through the skin, others can't be absorbed. Inhalation is the next best thing to injection, but probably not for every single one of them, so it's impossible to make broad statements about their efficacy and median lethal dose via different routes of exposure. The exact same thing applies to the "poisons" category - while something that is damaging when absorbed cutaneously, such as methyl mercury, is probably just as damaging directly injected into the bloodstream, it's impossible to say that with certainty about all poisons.
Most venoms contain a lot of enzymes. These enzymes can be split by the proteases in your stomach. So if they don't have any effect before reaching your stomach, they will probably digested like any other protein. Ofc. there are some proteins which resist digestion.
There are poisons which affect only certain cells in the digestive system, so injecting them probably would not have any effect or it can have a completely different effect. E.g. cholera toxin (if we can call it a poison) affect the glucose transport when injected, while it causes diarrhea when it is produced by the bacteria in the infected guts.
There are toxins which have the same effect from both ways. E.g. botulinum toxin can be deadly both injected and ingested. You probably heard of botox injections...
The effect can vary by delivery method as well. Injected can mean subcutan, intravenous, etc... while ingested can mean drinking, swallowing a capsule, which protects it from the proteases in the stomach, and so on... It depends on the dose as well.
I think MattDMo is right, the question is too broad because there are many different poisons and venoms and delivery methods.
To answer this question first we need to look at what is the definition of a venom and poison. This is somewhat difficult because the definition has changed slightly over the years, but looking over them we can learn of the similarities between the two.
"A venom is typically a mixture of compounds containing one or more toxins, which is produced in venom glands and injected into animal tissues via specialized morphological structures." This meaning that a venom is made and is injected into another organism (both plants and animals can do this). "A poison is one or more compounds that are toxic. These toxins can be inhaled, absorbed, ingested, or injected. Plants and animals can produced organic poisons; however, poisons can also come directly from the environment." Example: Chlorine or Fluorine gas.
This is where it's tricky with the definitions. A venom can only be injected to be harmful, whereas a poison can be as well; however, a poison has a broader area of effect. Some snakes have venom that can be absorbed or toxins in their flesh making them poisonous to eat. So the issues are first what happened, was it injected or not, next could it have not been injected and still caused harm.
The best logical answer is that all venoms are poisons, but not all poisons are venoms. This is because both can be injected and cause harm, but venoms have to be injected when poisons don't. Like with the above, some snakes have toxins that are commonly injected so they are still called venoms even when they can be absorbed making them poisons by definition.
To conclude, it's a complicated issue that people are still trying to figure out. In the end the main difference between the two is that a venom needs to be injected, when a poison can but doesn't have to be.
Some substances can be both venom and/or poison. According to Wikipedia, the source of all knowledge in this internet age, Blue Ring octopi are dangerous to humans if provoked because their venom contains the powerful neurotoxin tetrodotoxin. According to the same reliable source, Pufferfish are dangerous to humans if eaten because their flesh contains the powerful neurotoxin tetrodotoxin*.
*Summations, not quotes.