cbCalendar is the module view of the events and task, where you have list view, detail view and all the actions.
Calendar4You is the calendar view of these events and other modules with date fields
Calendar is obsolete and will be eliminated shortly. In fact we have already eliminated a lot of code in there. What is left is for use as web services external applications which should be able to use both modules but are recommended to move to cbCalendar which is where all future development will take place.