Area 51 is the staging site for recommending new Stack Exchange sites. On this site you can see information on all of the sites that have been proposed and their current state of creation (or rejection) along with examples of the kinds of questions that should be asked on the new Stack Exchange site so that an understanding of what is on topic and off topic can be established before the site reaches a commitment phase.
Super User is for questions regarding using computers, computer hardware, and home networking. Questions here are often about configuring and customising operating systems, problems and errors that occur when running applications and configuring home networks.
Stack Overflow is for professional and enthusiast programmers to get assistance with problems they're experiencing when writing code. Questions on this site often include examples of code that the asker is experiencing issues in along with answers containing potential solutions and advice.