Does it mean that we can write a RDF file to describe some resources, and we can describe the relationship between those resources by using OWL in an another file? So I should have the RDF file before using OWL?
If I want to use RDF to describe the web resource (a web page), the URI is should be the URL, is that right? But in that web page, maybe a lot of (more than one) resources need to be described, but the URL is only for the whole web page, in this case, what can I do for the other resources in this web page??
Does it mean that we can write a RDF file to describe some resources,
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RDF can describe both the resources, and the relationships between them.
OWL is (optionally) used to properties (relationship types) that you use in the RDF. Usually people reuse an existing ontology/schema (e.g. FOAF, DOAP, Dublin Core, SKOS, which are written using RDFS or OWL) to make it easier for other people to understand their data. If you’re describing something very specific though, or you need to go beyond a common schema, then you can invent your own properties, and describe them in RDFS or OWL if you like.
URIs do not have to be URLs, so if for example you want to describe the city of London, which is not a webpage, then you can invent a URI for it, or reuse an existing one, e.g. the DBPedia URI for London is http://dbpedia.org/resource/London – if you paste that into a web browser you will be redirected to a page about that URI, but the URI itself isn’t a URL.