I need to create a UML use case diagram based on use cases that I already have specified in an event table. The event table has a source column which in my case is the same thing that I would specify as the actors used in the use case diagram.
My question is what is the difference between the actor and the source?
They're pretty much the same thing.
Event tables are just another way of writing up a system, similar to a use case narrative. A use case narrative is a way of documenting the activity of a single use case element in a use case diagram. The event table can form the basis for a use case narrative; you can also add an activity diagram for a visual representation of what happens in the use case.
Event tables have events, triggers, sources, use cases, responses and destinations. Analogous sections of a use case narrative are triggers, which are sort of a combination of events and triggers (How the use case starts), actors, use cases, post-conditions and exit points. A use case narrative can also have entry points, preconditions, and assumptions (assumptions are not tested, preconditions are), includes and extension points, and will also have a detailed description of the steps on the paths through a use case.
