DoubleClick, through it's advertising program DART, is an internet advertising company. It is responsible for some of the pop-up ads you see.

Most pop-up ads are easily preventable by simple disabling Javascript in your browser. DART is specifically easy to opt out of by a simple procedure that will change DART's cookie (Go here to see how:

http://www.doubleclick.com/us/about_doubleclick/privacy/dart_adserving.asp