The only difficult difference here is Stack Overflow vs Programming, the others are pretty descriptive. The difference is not too hard to understand, but still not intuitive for a new user.
There is also some overlap, so if you have a question about Ubuntu you can also ask it on Unix&Linux.
For me, Stack Overflow vs Programming is easiest to understand - it's whenever the question is about implementation or theory. But some areas, for example, packaging, lie in the grey area between programmers' and sysadmin's job.
There is also some overlap, so if you have a question about Ubuntu you can also ask it on Unix&Linux.