What is the difference between run project and run file on netbeans ide 7.3?

What is the difference between run project and run file on netbeans ide 7.3?

I was working on a JApplet program, then i pressed the hot key, F6 ( run
project). It gave me an error, then i pressed Shift + F6 ( run File),
which worked. So my question is, what is the difference between run
project and run file on netbeans ide 7.3?
This is the code i was using.
package javaapplication128;
import java.awt.Graphics;
import javax.swing.JApplet;
public class JavaApplication128 extends JApplet {
public void paint(Graphics g) {
super.paint(g);
g.drawString("hello", 25, 25);
}
}