When you access DuckDuckGo (or any Web site), your Web browser automatically sends information about your computer, e.g. your User agent and IP address.

Because this information could be used to link you to your searches, we do not log (store) it at all. This is a very unusual practice, but we feel it is an important step to protect your privacy.
https://duckduckgo.com/privacy

Now there’s a reason why other search engines don’t do the same – they need to analyze visitors and they need to monetize their product. DDG point out that IXQuick.com is another search engine that takes privacy seriously.

Both sites still manage to have contextual search ads that provide them with an income. What they can’t do is personalize your results, which is slowly and surely becoming more important. But on the other hand, the SERPS remain the same for everyone, something that webmasters tend to prefer.