In contrast to the bannerexchange, where graphics (banners)
are exchanged, pop ups work with sites.
The user exchange the visitors among each other.
If the visitor accesses a participating website another window
opens up containing the website of another user.
There are two different kind: The intro- and exit up exchange.
The window containing the intro pop up opens up just after
the site has been accessed and the one containing the exit
up after the site has been left.