Class B, C, and D Airspace are designated for that controlled airspace around airports where there are minimum requirements for communications and/or equipment and pilot certification.
Basically, Class B airspace exists at the nations busiest airports; Class C and D are for airports with operational control towers (Class C also having radar approach and a certain number of IFR ops. or passenger emplanements).
Look here for more info:
http://www1.faa.gov/ATPubs/AIM/chap3toc.htmMost everything aviation-related can be found in the good old AIM!