I have a task to investigate a file loading process in a one of our company software, I have to represent main classes and methods and what kind of work is done in that and finally write a report about that with some kind of diagrams.
what is the best way to describe this workflow by writing text document or by using UML and if UML which kind of diagrams should I use?
Use UML Activity Diagram (http://www.agilemodeling.com/style/activityDiagram.htm) for the flow of the processes at a high level.
Then, use UML sequence diagram (http://www.agilemodeling.com/artifacts/sequenceDiagram.htm) for portions complex enough to warrant further detail.