General Concepts of links and association
A link is a conceptual or physical connection among objects for instance. Mathematically, you can define a link as a tuple which is an ordered list of objects. Moreover, a link is also defined as an example of an association. Or we can say that an association is a group of links with a common structure and common meanings, for instance, a student study in a university. All links in an association connects objects from the same classes. A link is used to show a relationship among two (or more) objects.