[SEAPY] Python diagramming tools?

[R Fritz]

I just had occasion to prepare some class diagrams of a Python app  
yesterday, and I discovered that I didn't know what tools could be  
used to do this on a Mac and I didn't know graphical practice,  
anyway.  PyUt, which appears to be a useful diagram generator, is so  
far out of date that it depends on no longer supported methods and  
packages.  So I came away with these questions:

1. WtF is UML & do I need to learn more about it?
2. Are there tools which generate UML models from Python code?
3. What are good general purpose cross-platform diagramming tools?
Help?

Randolph