If you want node to node relations, this is the module for you. In essence its a module that only allows you to create node relations and presnet them in simple lists. With each node, you can define a parent, the new node will then become a child of that parent. In this way, you create parent-child trees, like book module does, in an easier and more flexible way. The APIs are more flexible, the database structure is very flexible and the concept is thought out to suit more cases then just 'books'.
The name comes from a paperclip, that can clip several papers together.