Airspace is split in to different classifications, each with its own purpose and each with its own rules. There are seven classes of airspace, lettered A through G. There is no Class F airspace in the United States, so you will not need to learn about Class F.
In addition to its classification, airspace may be controlled or uncontrolled, towered or non-towered.
Controlled Airspace
Airspace is controlled where air traffic control services are provided. Class A, B, C, D, and E airspace is controlled.
Uncontrolled
Airspace is uncontrolled if air traffic control does not exercise control of aircraft within the airspace area. Class G airspace is uncontrolled.
Towered / Non-Towered
These terms refer to the presence of an operating air traffic control tower at a particular airport.
Height References
Before discussing airspace, it is important to be familiar with general differences between height references.
MSL: Height above the mean sea level. This is the altitude that displayed on the altimeter.
AGL : Height above ground level
FL : Instead of maintaining a constant height above mean sea level, the pilots flying above 18,000 feet maintain a constant flight level. In order to maintain a flight level, pilots set their altimeters to the standard altimeter setting of 29.92. Flight levels are referred to by the first three numbers of the height to be maintained. For example, if told to maintain FL230, a pilot would maintain 23,000 feet, with the altimeter set to 29.92.
Class F airspace is foreign.