Specifically we are talking about browser windows.
When you click on a link, sometimes you want the new page to appear in place of the current page.
At other times, though, you want the new page to appear on its own.
What about the features of the new window?
You may want to control --- its size or location, whether the scoll bars appear or not,
or whether the visitor can resize the window
Small pop-up windows are good for hints and reminders.
They can appear upon clicking on a link, or when a visitor first displays a page.
You can have pop up windows that close automatically,
move around the screen, or shake in place!