Difference between ARN and uri

I hope I can shed a little light into this topic.

The Amazon Resource Name (ARN) is used to uniquely identify AWS resources. A Uniform Resource Identifier (URI) provides a name to an accessible resource.

In S3, the URI is a resource identifier within the context of the S3 protocol. It is used for API access to said files. There is some more details regarding this here on an already answered post.

In a summary of the differences, an ARN is used within the AWS Cloud infrastructure to identify/access resources, while the URI is used externally identify/access these resources via APIs.