Over the last two years, we’ve seen companies develop display solutions that show ads based on the keywords prospects search as they navigate the Web.
The introduction of search retargeting represented a major leap forward for direct response and brand marketers who were thirsty for new audience acquisition and engagement solutions that performed better. What they got was transparency and insight into the data used to reach consumers — they could use consumers’ actual online searches instead of the 3rd party data shoved into a fixed, black box audience segment that had become the norm. 
Early leaders in search retargeting (including my company, Simpli.fi) have developed the technology to execute keyword-level search retargeting with display ads.
In addition, legacy demand side platforms have attempted to replicate the service with more fixed segments from 3rd party data providers; a small crop of managed media solution providers are beginning to market search retargeting to their clients; and most, but not all, are leveraging one or more of the previously-mentioned technologies or tactics.