Sequence diagrams depict objects and their lifelines. When an object becomes active, the lifeline may be widened to a rectangle for the period of activity.
Collaboration diagrams depict the objects and their relations in a more static manner. Messages between objects, indicated by arrows, may be specified in greater or lesser detail. Also here, as a rule of thumb, details are given only when needed. To indicate message sequencing in collaboration diagrams numbering may be used, increasing or, as illustrated in slide uml-interact(b), branching.