*where can I found topics for every site of this network and differences between [superuser][1] and [serverfault][2].*

This is perfect network where you can find answer for almost question you need. and it's free of charge. These reasons are big plus to use it. 

but any way, why [this](https://stackoverflow.com/questions/1018078/testing-html-email-rendering) is on StackOverflow and [this](https://superuser.com/questions/349834/installing-multiple-versions-of-ms-outlook/) is on SuperUser? 2 duplicates.

And there's much more same examples to show.

  [1]: http://superuser.com
  [2]: http://serverfault.com