Diagrams, yeah. I dunno if it's officially UML or not. I've never been all that concerned with UML actually. I haven't ever really found it useful, and neither have most of the people I've worked with. Usually you just dive right in with some use cases and build some unit tests for them, and build the API around that, and then fill in the internal details to match the API and get the tests to pass. Or at work we just sort of hack on the source and fix things when clients complain