Pop-Ups
Recommendation: Don't use pop-ups.
Why?
Unless pop-ups are specifically optimized to work on mobile devices and tablets, they don't look good, and it can be difficult to close the overlay. However, they are useful in concentrating a user's attention in a small area. Pop-ups can allow a user to become familiarized with new content.
Pop-up advertisements interrupt a user's activity and are not recommended. However, displaying a helpful introduction within a pop-up window could improve user experience. Ideally, the user would stay on the same page, and the pop-up would be displayed as a small overlay.