Amazon S3 is the storage system from where you can upload/download files (from normal HTTP requests). S3 is organized with the idea of buckets from which you may store/download files. Also, there are plugins that allow you to map S3 directly to your EC2 instance like some remote folder (which is not recommended):

The following screenshot shows how to create a bucket:

The S3 system can be configured publicly, so people can download/upload stuff from anywhere.