An association describes discrete connections among objects or other instances in a system. An association relates an ordered list (tuple) of two or more classiﬁers, with repetitions permitted. The most common kind of association is a binary association between a pair of classiﬁers. An instance of an association is a link. A link comprises a tuple (an ordered list) of objects, each drawn from its corresponding class. A binary link comprises a pair of objects.
Self Association also called reflexive association, that is, A single object may be associated with itself if the same class appears more than once in an association. If the same class appears twice in an association, the two instances do not have to be the same object, and usually they are not.