We got fancy with the CSS Opacity property type of pop-up, which is a pop-up that slowly appears on the page. There is a lot more code to make this work than ought to be needed. But this is a classic case of each browser maker deciding to go his own way. Worse yet, IE 6, IE 7, and IE 8 all have different coding needs for this feature. The result is that we had to state the opacity codes 4 different ways, to try to please everyone. But, in spite of all the over-coding, with Windows Opera it was no good—nothing
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