What is up with these links?

The ones I am referring to are the ones that are above the threads in these forums that are specific to the topic of the forum/thread. They change with each forum. They also can be found when you do a Google search and are usually above or to the side of the results in smaller print.
Whenever I click on most of them I get a blank screen with the words at the top on the blue bar which reads "ADCLICK 32x32 GIF" That is all I get. Does anybody have any info about this. It's annoying when the link looks good and you click on it and get that message.