To hear Ethan Zuckerman -- an early Tripod employee and current head of MIT's Center of Civic Media -- tell it, Tripod's bread and butter became analyzing the sites people created and targeting ads for them. The traditional banner ad could be a tricky proposition for a brand, though, since they could imply some sort of corporate endorsement of whatever's on a given site (Zuckerman mentions a car company mortified to see one of its banners on a site discussing anal sex). That's where the pop-up came in: they allowed ads to appear on a site without actually being on the site. Turns out, Zuckerman was instrumental in making these terrible things, well, a thing. To quote his statement to Wired:
Little did Zuckerman and company know that they inadvertently released a scourge upon the rest of the internet -- as they say, the road to hell is paved with good intentions.