I think the US has multiple staple crops, but corn is probably the biggest (after maybe wheat; possibly vice versa). Corn is used in corn syrup (high fructose or otherwise) and many corn-related foods, as well as in corn starch, a fairly common additive (I think). Wheat is used in most forms of bread in the US, as well as in many other places flour is necessary. My parents generally try to avoid corn syrup in our food, although that's not always particularly successful given how ubiquitous it's become. We eat corn on the cob and probably lots of things with corn starch. For wheat, most of our bread is made of it, which includes things that are partly bread like pizza.

Different cultures usually have different staple crops for multiple reasons. One is availability of seed; if a plant is native to an area, it is more likely to be available for growing than if it's imported (at least at first). Another is conditions; some plants grow better in a certain climate than others.

If a culture relies too heavily on a single staple crop, terrible things can happen if anything hurts that crop. The classic example is the great Irish potato famine, when the whole country suffered because a single disease had wiped out most of the potatoes, all being the same breed.