You can choose who gets to see which of your tunes and documents
When you've created a tune or a document, you have complete control over who has access to it.
By default, tunes are publicly visible, documents are only visible to their owners, and both types of object can only be modified by their owners (in the case of documents, editing is not the only possible modification; one can add tunes into it, or make it be the "parent" of another document). If this isn't what you want, you can change it. This is worth paying attention to - I daresay many people will have stuff that they don't want to show to the whole world, copyright being an important one.
You can also keep private notes on the tunes you do make publicly visible
The 'PrivateNotes' field of a tune is only visible to people that have permission to edit it.