Airspace Notes

Pandemonia  has lots of other interesting and useful information about "real" flying, especially in the Bay Area.

Class A airspace is tightly-controlled, instrument flying only, and above 18,000 feet. This tends to be where the jets are while cruising....

Class B airspace surrounds the biggest airports (like SFO). Class B airspace is tightly controlled, and usually very busy. Student pilots can't fly in Class B airspace without their instructor's explicit written endorsement [which I now have... HR].
Class C airspace surrounds busy airports like Oakland or San Jose. It's not quite as tightly controlled as Class B, but still has radar, full ATC services, etc. They let people like me fly in Class C.
Class D airspace surrounds smaller airports (like PAO or LVK) with a control tower, but not (usually) radar. Often very busy. Often scarier than class B or C because of the lack of radar....
Class E airspace is all controlled airspace that isn't one of the above. Most of the US airspace below 18,000 feet and not one of class A, B, C, or D is class E.
Class G airspace is uncontrolled airspace, typically just the lowest altitudes away from airports. There's not a lot of it in the Bay Area...."

Class B airspace generally looks like an upside-down wedding cake.