What do the green, white and yellow bands mean on an airspeed indicator?
The FAA has a nice introduction document on flight instruments, which explains all the standard markings on a typical airspeed indicator (ASI):




  As shown in [the figure above], ASIs on single-engine small aircraft
  include the following standard color-coded markings:
  
  
  White arc—commonly referred to as the flap operating
  range since its lower limit represents the full flap stall
  speed and its upper limit provides the maximum flap
  speed. Approaches and landings are usually flown at
  speeds within the white arc.
  Lower limit of white arc (VS0)—the stalling speed
  or the minimum steady flight speed in the landing
  configuration. In small aircraft, this is the power-off
  stall speed at the maximum landing weight in the
  landing configuration (gear and flaps down).
  Upper limit of the white arc (VFE)—the maximum
  speed with the flaps extended.
  Green arc—the normal operating range of the aircraft.
  Most flying occurs within this range.
  Lower limit of green arc (VS1)—the stalling speed
  or the minimum steady flight speed obtained in a
  specified configuration. For most aircraft, this is the
  power-off stall speed at the maximum takeoff weight
  in the clean configuration (gear up, if retractable, and
  flaps up).
  Upper limit of green arc (VN0)—the maximum
  structural cruising speed. Do not exceed this speed
  except in smooth air.
  Yellow arc—caution range. Fly within this range only
  in smooth air and then only with caution.
  Red line (VNE)—never exceed speed. Operating above
  this speed is prohibited since it may result in damage
  or structural failure.
  


(FAA Handbook - Chapter 8 - Flight Instruments, emphasis mine)