Airspace Classes

From the Wikipedia: http://en.wikipedia.org/wiki/Airspace_class

• Class A: All operation mu be conducted under Instrument Flight Rules (IFR) or Special visual flight rules (SVFR)
• Class B: Like Class A but can also Visual flight rules (VFR)
• Class C: Like B but flights can be separated by VFR.
  

See reference for rest of classes.

A-E are controlled airspace.

F and G uncontrolled airspace.