1. What is the differences between using #include "qwidget.h" or #include <QWidget> or #include <qwidget.h> ?
Not much. The first will search the current directory and then the standard directories for a file called "qwidget.h" and include it. The second and third options will search for the named file only in the standard directories. This is standard C/C++ pre-processor behaviour, nothing specific to Qt.
http://en.wikipedia.org/wiki/C_prepr...ncluding_files
2. I wrote a small app. How can I run it under other windows machines which haven't got qt library. Which dll's should I copy? How can I use them ?
http://doc.qt.nokia.com/latest/deployment.html