Custom Ringtone Maker is an app that allows users to create "chaku-uta" (ringtone) from songs in their iPod library when used in conjunction with a computer. Users can easily create ringtones using songs from their iPhone's iPod library and set them for phone calls, emails, and SMS notifications.

Please note that in order to register the created ringtones on an iPhone, users need to connect their iPhone to a computer and configure settings in iTunes. Creating ringtones from songs purchased on iTunes may not be possible due to rights issues.

The app's functionality is limited to creating the ringtone file. Additional steps such as registering the created ringtone in iTunes and syncing the iPhone to a PC or Mac are required separately.