How to download Audio files from You tube video

Hello friends.......
Today I am going to tell you how to download the audio file of you tube video file.
that means we can download the any you tube video file in audio format that means mp3.
because some time we really like to listen the audio or mp3 according to video file.
so that time we use this trick to download the audio file from you tube.
Actually we can download any You Tube video