I'm trying to understand the difference between Super User and Stack Overflow. What kind of questions should I ask here and not on Stack Overflow?
Stack Overflow is for programmers, Super User is for computer hardware & software enthusiasts and power users. You want to ask questions about code, go to SO. If you're having a problem with booting your laptop, go to SU.
Read the FAQs of both SU and SO to understand more.
The Stack Overflow Trilogy
SO is about any way of creating Software. There are many ways of creating software, wether it's programming, scripting, or even the drag and drop from the Lego Robot.
Meanwhile Superuser is more about Computer questions from the end-user. If I have a software problem, of if I am just curious of how something is done by the computer (how does a cpu know what commands and instructions actually mean) this would be the site to go