A resource is any "thing" that can be referred to using a URL or a path. It can be a physical device, a logical pseudodevice, a file on a file system, a service that has a name, or an element of a dataset.
The client program accesses a resource by opening it, using the resource name in URL format. The first part of the URL is the name of the scheme, and the rest of the URL is interpreted by the scheme provider, assigning whatever meaning is appropriate for the resources included under that scheme.
TODO Give good examples of resources