A link is an instance of an association. While an association connects classifiers to show structural relationships, the link (association instance) connects classifier instances. It is a list of the references between classifier instances. Its most common expression is a reference to two classifier instances in its binary form: an n-ary association will have n-ary links. Properties such as aggregation can be used with a link, however multiplicity and ordering may not as they have no meaning.
Explanation
A link is a specific example or instance of an association. While an association describes the structural relationship between classifiers from a generic point of view a link describes or represents an actual or specific example of this relationship. In our natural language we use this idea regularly when we talk about concepts generally. For example we could talk about the concept of a person owning a pet and an instance of this would be a particular named person owning a particular named pet. This is part of what is known as the type-instance dichotomy, which occurs with many of the UML elements.
An object is related to an association instance because they are used together. Instances of classes (objects) are connected to each other by (links) instances of associations. These are an expression of actual links between the objects and represent instances described by the structural association relationship.
