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.
StackOverflow is about programming. Superuser is about using the computer in general, actually more about software than hardware.
The Stack Overflow Trilogy