Soybeans are a legume that grow about 8" high. They cover more the 75 million acres in the US and gross more than $18 billion a year making soy the second biggest planted field crop. (For a little perspective, New Mexico is 73 million acres and the porn industry sells $20 billion in adult movies).

Here's a link to basic soybean stats. And here's a map of where soybeans are grown (mostly along the Mississippi and Arkansas rivers).
The soybean industry is quick to call soy a miracle crop that accounts for 64% the world's protein consumption. They are, as they say, "the world's foremost provider of protein and oil." I wouldn't know how to qualify that other than saying that Tyson Chicken also call themselves "the world's premiere protien provider," and Damien in the movie Omen also called himself the leading provider of protein.
Nevertheless, US soybean production has increased 33% in the last 10 years thanks to China doubling it's soybean imports to to be used for humming meat consumption. In fact, China is the only country buying more and more American soy exports, roughly $2.5 billion or 36% of all exports and 13% of US soybean production.Another reason for the increase in soybean production is biofuels (more to come soon).
All this being true, soybeans are loosing ground to corn and overall soybean prices are about the same as they were 30 years ago (though there has been a 23% jump in soybean oil prices--again, more to come soon). For 2007 crops corn is expected to out-earn soybeans; estimates say soybeans will earn nearly $100/acre than corn thanks mostly to increased demand for corn-based ethanol.