AIDL : Android Interface Definition Language
Using AIDL is necessary only if you allow clients from different applications to access your servivce for IPC and want to handle multithreading in your service.
If concurrent IPC across different applications is not required then create interface by implementing a Binder.
If IPC is required but multithreading is not required, then implement your interface using a Messanger.
Binder and Messanger are implemented in android.