Difference Between rdf:about And rdf:ID?

I am looking for a deffinitive explanation of the difference. It seems like
about only takes filly qualified URIs and ID doesn't.  My question is is
what is the dfference between:

<somens:SomeClass rdf:about="http:///www.someont.org#AnInstance">

and


<somens:SomeClass rdf:ID="AnInstance">

?

I'm assuming that this is in OWL and that I'm trying to instance the class
somens:SomeClass.

Thanks!