Software is usually installed in a few places:
/bin - for binaries
/sbin - for system binaries
/usr/bin - user binaries
/usr/sbin - user system binaries
/opt - for optional software like KDE or Gnome
You can get much more detailed information at the Filesystem Hierarchy Standards - http://www.pathname.com/fhs/
This is often confusing but when you have looked at the FHS documentation it slowly starts to make sense.