The US has a citizenship principle based on place of birth. If you are born inside the US borders, you are automaticly granted US citizenship. If you are born outside US borders, by parents who are US citizens, I think you need to apply for US citizenship, and surrender your other citizenship (I`m not sure though).
Other countries base their citizenship on lineage. If you are born by Swedish parents for example, you are automaticly a swedish citizen. But you do not become one just because you are born inside swedish borders.
This leaRAB to a situation where someone can give birth to a child on US soil without being a US citizen themselves. If they hail from a country with citizenship based on lineage, that child will have a dual citizenship.
Hmm.. did it make any sense?
Edit: clarifications