So here's the deal. People from Chicago and a good deal of the rest of the country seem to think that everything south of I-80 constitutes "southern Illinois". This is simply not true. And most people in other parts of the state will find it downright offensive. So listen up, boys and girls, I'm going to tell you a story about the great state of Illinois.
Illinois is composed of no fewer than four distinct cultural and geographic regions. Six, if you count greater Chicago and East St. Louis as separate from Northern and Southern Illinois, respectively. Let's work our way up from the bottom up, shall we?
Southern Illinois, that is, the real Southern Illinois, starts approximately south of I-70. For those of you who don't know your highways, cut a line through Illinois that runs from Indianapolis to St. Louis. Here's the thing with Southern Illinois. Some 12,000 years ago, there was this Ice Age, see? Towards the end of it, there was a glacier covering a good deal of North America. Most of Illinois was covered by ice, but not Southern Illinois. So, while the other parts of the state were ground flat by ice and rock, Southern Illinois has these things called hills, trees, and even the occasional canyon. Lakes, too. In fact, a good portion of Southern Illinois actually lies within Shawnee National Forest. Can't say that about Central or Northern Illinois. You Chicagoans may not even know what trees are. And yes, there are still farms in Southern Illinois. But where corn dominates Central and Western Illinois, those in the south grow more soybeans. As noded by Gorgonzola, Southern Illinois is sometimes called Little Egypt.
East St. Louis is a four or five county region home to half a million people. It deserves separation from Southern Illinois, not just because of a higher population density, but due to its dependence on nearby St. Louis. While East St. Louis itself is but a shadow of its former self (having one of the highest crime rates in the country), much of the surrounding metro area is actually quite pleasant.
Central Illinois is everything east of the Illinois River, north of I-70, and south of I-80 (and west of Indiana). This is what you are probably actually thinking of when you use the term "southern Illinois". Miles and miles of corn. Some big cities like Peoria and the capital Springfield that Chicagoans laugh at for considering themselves "big cities" (Peoria being the second largest metro area in Illinois) spotted here and there. For you Chicagoans wondering if the Dan Ryan counts are a prairie, this is where the name "Prairie State" comes from. This is where they grow the corn. But before you dismiss Central Illinois as a backwoods farmer's hickland, the region is also home to one of the top research institutions in the country, the University of Illinois.
Western Illinois is bordered on the west by the Mississippi River and the east by the Illinois River. While similar to Central Illinois, it deserves mention due to its borders. Until the 1980s, there were precious few river crossings connecting Western Illinois to the rest of the state. In the 1960s, it earned the nickname "Forgottonia" due to its isolation.
Northern Illinois is everything but Chicago north of I-80. They do their fair share of farming here, too, but there area is much more culturally tied to Chicago than the rest of the state. They root for the Bears and the Cubs (or White Sox) instead of the Rams and Cardinals. You'll find Rockford in Northern Illinois, which is the second largest city in the state and is quite distinct from Chicago. There's also a small section of northwestern Illinois which was spared from the Ice Age glaciation, and bears some resemblance with hilly Southern Illinois. The highest point in Illinois above sea level, Charles Mound, is here. And no, though higher, the Sears Tower does not count.
Chicago city is home to three million people. An additional three million reside in the 'burbs. This gives it roughly half the population of the entire state. Frankly, I have very little to say about Chicago. The Chicago writeup says more than I could hope to in my little rant, so check there for information about the windy city.
That's the end of my rant. But remember, oh Chicagoan, do not dismiss 95% of your state's area and half its population as "southern Illinois". Yes, we are different from you, but we are also quite distinct from each other.