Is there such a thing?

I swear Ive seen it on certain websites. But I didnt know if this was a thing put in by the web developer. Or its was in fact an affiliate ad that self displays relevant add based on page content.

Couldnt find anything on it, figure I ask.

thanks in advance

It's doable but very difficult to get the targeting correct unless you work in a tight niche.

A lot of people have an affiliate data feed (eg. an Amazon feed) in a database and then they return ads based on incoming search keywords. There are hosted solutions like the ad boxes that Chikita has. There are also PPC feed providers that return ads based on search keywords (eg. what Peakclick used to do, I dunno if they're still around).

The problem is that some products just don't convert and you're better off sending your traffic to a related but better converting product, ie. manually selecting and targeting your ads. Personally I've always made the most money working in tightly niched sites where I know the surfer can be sold on a particular high converting product.