Confirm boxes will return "true" if ok is selected, and return "false" if cancel is selected. Alert boxes will not return anything. Prompt boxes will return whatever is in the text box. Note: prompt boxes also have an optional second parameter, which is the text that will already be in the text box.
Make a variable
test set it equal to a prompt box, and type "Hi!" in it (without the quotes) when it pops up. Note: your pop-up blocker must not be enabled.