From W3C Wiki
Ontaria helps you investigate Classes and properties. But, it won't look at arbitrary graphs. For instance, you can't put your FOAF file in there, and look at it with Ontaria. But you can put the FOAF specification in there, and browse that.
You'll note that it doesn't just stop with the file that was read- it cross references across the Internet (NetworkedData.) So you'll see WordNet information in there if you lookup Person, or something like that.
I don't fully comprehend Ontaria at the moment.
"What does it look at? What won't it look at?"
It seems to look at classes and properties. So if you feed it an RDFS description of some classes, it'll start reasoning over them.