A communication diagram, formerly called a collaboration diagram in UML 1.x , is an interaction diagram that shows similar information to sequence diagrams but its primary focus is on object relationships. It models the interactions between objects or parts in terms of sequenced messages.
A communication diagrams use the free-form arrangement of objects and links as used in Object diagrams. The messages are labeled with a chronological number and placed near the link the message is sent over. Reading a communication diagram involves starting at message 1.0, and following the messages from object to object.
A Communication diagram shows a lot of the same information as a sequence diagram, but because of how the information is presented, some of it is easier to find in one diagram than the other. Communication diagrams show which elements each one interacts with better, but sequence diagrams show the order in which the interactions take place more clearly.
UML has four types of interaction diagrams as show in the Figure below: