The Directed Edge REST API is a fairly simple way of modeling a collection of items and the relationships between them. These relationships are used as the basis for finding similar items or delivering personalized recommendations to a user.
Everything is an item
A product is an item. A user is an item. A web page is an item.
Items have tags, which is the usual way to differentiate between them. So a user is just an item with the tag "user". A product is an item with the tag "product".
You can create item types in an ad-hoc fashion, and items can have multiple tags, so you can also have an item with the tags product and sci-fi, for example.