ontologies are data with corresponding metadata about everything on a domain or site.
These ontologies allow the data to be parsed automatically, i.e sites like amazon could just make their data available as ontologies and third party sites could "read through the sites for us". These third party sites serve as a portal from where we could ask for what we want and they would in turn look through the sites available (eBay, amazon etc) using the ontologies and get the best deal for us.