One way to get a user interested in something is with an embedded video set to autostart. I prefer ALL videos to sit there and allow me the choice about whether to play them or not, and so do most people. But the advertisers lately have shown us no such respect—they have videos playing off to the side trying to get us to spend money on something we do not want and do not want to hear about or see, and it severely interferes with scrolling the page downward to see content. So, you just might want to refrain from setting it to autostart, in your web page designs. I noticed that even though most browsers loaded the video just fine, Windows Opera gave a message that said that a plugin is needed. Happily, some of the browsers did what we all hoped they'd do: ignored the autostart and waited until we either clicked the Play button or double-clicked on the video before it would play.
You may use this technique to present pop-up ads, or to solicit subscribers to your ezine, newsletter, or RSS feed. Use if for reminders, warnings, or statistics displaying. You may get survey participants, ask for donations, or use it for political fund raising. You can even inform visitors about some of your other related websites. There is no end to the types of purposes that pop-ups can be used for.
If your pop-up is about thumbnail enlargement, check out What is the easiest way to do thumbnail image enlargement windows?
If you want thumbnail enlargement without clicking, check out Making an Image Enlarge When The Mouse Cursor Is Hovering Over It