A retargeting DSP is a DSP owned by a retargeting specialist. Retargeters allow their clients to fetch back their visitors through the DSP. For more details on "classic" DSP see DSP definition.
The DSP capabilities are very important in retargeting process. Retargeting can be done at scale and in time if the DSP has access to major ad exchanges. Retargeter’s DSPs have also dedicated features for generating and optimizing personalized creatives tied to product catalogues.
Nowadays, all DSPs have retargeting capabilities, but specialized retargeting DSPs might have precious optimization features.