Connectors illustrate communication links between parts to fulfill the structure's purpose, typically in a Composite Structure diagram.  Each Connector end is distinct, controlling the communication pertaining to its connecting element. These elements can define constraints specifying this behavior. Connectors can have multiplicity. 

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OMG UML Specification

The OMG UML specification (UML Superstructure Specification, v2.0, p. 163) states:

"Specifies a link that enables communication between two or more instances. This link may be an instance of an association, or it may represent the possibility of the instances being able to communicate because their identities are known by virtue of being passed in as parameters, held in variables, created during the execution of a behavior, or because the communicating instances are the same instance. The link may be realized by something as simple as a pointer or by something as complex as a network connection. In contrast to associations, which specify links between any instance of the associated classifiers, connectors specify links between instances playing the connected parts only."