JSON & Related Fields
Custom Fields offers JSON fields for developers in several places throughout the app.
The JSON Field in the app is simply a place to store and edit raw JSON data. This is most useful in custom code implementations.
The content entry form is simply a text field. There are no configuration options and the template generator just prints the JSON.
Field Collections & Widgets
Field Collections & Widgets in Custom Fields use JSON fields in the underlying architecture and are much more "user friendly".
Field Collections are JSON fields that any other field type can be attached to in the interface. For example, a Field Collection for books might contain a Text field for the author and and an Image field for the cover image. All data for the author field and the cover field are stored in one JSON metafield in Shopify. The app can create templates automatically. A Field Collection template will automatically loop over all the fields and print them one after another. This makes an excellent starting point for customizations.