An airband scanner enables you to listen in to the aircraft transmissions.
The VHF aircraft band (108-137MHz) lets you listen in to civilian aircraft transmissions while the UHF aircraft band (225-400MHz) lets you listen in the military aircraft transmissions.
If you are only interested in an airband scanner for listening in to civilian aircraft at your local airport then the Uniden UBC-72XLT or Maycom AR108 are quite popular.
One of the best handheld airband scanners that you can buy today is the Uniden UBC 3500XLT which has a comprehensive list of features and includes the civilian and military aircraft band.
The Uniden UBC-125XLT is cheaper than the UBC 3500XLT but is very good value for money and also includes the civilian and military aircraft band.
More information can be found at Airband Scanners.