RDF vocabularies

Hi,

I am new to RDF and have a question about  schema/vocabularies.

Is it true that a namespace uri in rdf must point to a valid schema
definition ? An XML analogy
could be that I have to make sure that a DTD is available before parsing
the XML doc, correct ?

Also, is there a location where I can find out what existing RDF
vocabularies are available which
I can use instead of reinventing the wheel. 

Thanks,
Stan