Moving one step further, the next level of abstraction considers the relationship between two interacting objects. Typically one of the objects if providing a service, and the other is making use of the service. The important feature to note here is the structure of the interaction between the two agents.
By the way, we use the term client and server here in a very loose and informal way, not necessarily as the terms are used in, for example, the networking sense. Although network clients and servers can be viewed as a special case of this relationship.