A connector is a feature that is used in the description of composite structures. It specifies that two or more instances are able to communicate with each other in some way. The connector does not specify how the communication is achieved but simply that it is possible. In practice the instances may be able to communicate because there is a link (instance of an association) connecting the elements or because one instance has access to another through parameter passing.
Instances have the ability to communicate with each other and when a system is implemented this can be achieved by a variety of means. The connector is a way of specifying the ability of one or more instances to communicate without specifying how the communication is achieved. Connectors are used with instances and have no application to classifiers.