Native extensions for Adobe AIR are a combination of ActionScript classes and native code that provide easy access to device-specific libraries and features that are not available in the built-in ActionScript classes.
A Native Extension allows native code to be packaged along with an Actionscript application practically extending AIR. The code in the extension can utilise native components on the device and provide an Actionscript API to access these components from your application. "Native extensions provide easy access to device-specific libraries and features that are not available in the built-in ActionScript classes."
They are a combination of ActionScript classes and native code in a single ANE
file.
The Adobe maintained list: http://www.adobe.com/devnet/air/native-extensions-for-air.html