DX
The dx tool lets you generate Android bytecode from .class files. The tool converts target files and/or directories to Dalvik executable format (.dex) files, so that they can run in the Android environment. It can also dump the class files in a human-readable format and run a target unit test. You can get the usage and options for this tool by using dx --help.
This tool was placed in : %sdk%\platforms\%androidplatform%\tools\
AAPT
aapt stands for Android Asset Packaging Tool and is included in the tools/ directory of the SDK. This tool allows you to view, create, and update Zip-compatible archives (zip, jar, apk). It can also compile resources into binary assets.
Though you probably won't often use aapt directly, build scripts and IDE plugins can utilize this tool to package the apk file that constitutes an Android application.
This tool was placed in : %sdk%\platforms\%androidplatform%\tools\
AIDL
AIDL (Android Interface Definition Language) is an IDL language used to generate code that enables two processes on an Android-powered device to talk using interprocess communication (IPC). If you have code in one process (for example, in an Activity) that needs to call methods on an object in another process (for example, a Service), you would use AIDL to generate code to marshall the parameters.
The AIDL IPC mechanism is interface-based, similar to COM or Corba, but lighter weight. It uses a proxy class to pass values between the client and the implementation.
(In detail at Android - aidl)
(Refer from Development Android)
