A swimlane diagram (also sometime called a cross-functional diagram) documents the steps or activities of a process flow or workflow.  More specifically, a swimlane diagram groups these activities into swimlanes which are horizontal or vertical columns that contain all of the activities which fit into the category represented by that swimlane.  Swimlanes can represent many categories of information such as actors which perform the activities (i.e., role or department), the stage of the process in which the activity takes place, or whatever else the creator of the document feels should be emphasized and communicated by the swimlane diagram.  The term swimlane was adopted due to the visual similarity between the horizontal rows of the diagram to that of the swimlanes found within a swiming pool.

A process flow or workflow diagram does not have to use swimlanes.  However, since swimlanes communicate additional information about who performs the activity or when it takes place, it’s typically a preferred best practice to include them.  Similarly, a swimlane diagram can use only one set of swimlanes (either vertical swimlanes or horizontal swimlanes).  In BPMN standards, a process flow is typically shown flowing from left to right.  If only one set of swimlanes is used, horizontal swimlanes are most commonly used to group activities.  In the UML standard, the activity diagram flows from top to bottom and veritcal swimlanes are most commonly used.   

One of the more common choices used by creators of swimlane diagrams is to define the roles which perform each activity within horizontal swimlanes and define the process stages in which the activity occurs within vertical swimlanes.