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