Class C airspace is "crowded," meaning there are many aircraft trying to take off or land and requiring assistance, but not as much as at Class B airports. Class C airports include Sacramento, California; Boise, Idaho; Portland, Oregon; and others.
Class C airspace looks like an upside-down two-layered cake. Most Class C airports have a 5-mile radius around the airport that goes from ground level to 4,000 feet AGL, then a larger 10-mile radius above it going from 1,200 to 4,000 feet AGL. If there is another airport nearby, the two airspaces can take nibbles out of each other so they both look like partially eaten cakes. You'll learn how to determine the size and shape of controlled airspace as you learn to read sectional maps in other guides.
Class D airspace typically extends from the ground to 2,500 feet AGL above other airports with a control tower. Even if you have an endorsement for entering this airspace you're not supposed to do so unless you get permission by calling on your communication radio. You start a dialog with the controllers.
As you can imagine, there are many more Class D airports than there are B or C airports. In fact, you'll find many of them underneath Class B or C airspace. It can get confusing. Fortunately, the communication frequencies for these airports are widely published and you must know them as you plan your flight.