A node ref is a primary key for a node.
An association is a link between two nodes. The association has a name and a type and contains two node refs, one for the parent and one for the child. In addition an association can be primary or secondary. A node has one Primary association and will cascade delete the child on deletion of the parent. Secondary associations are aggregations and are many to many.
In your case the relationship between the article and the author is probably some sort of secondary association.