WHY YOU SHOULD CARE
At other search engines, when you do a search and then click on a link, your search terms are sent to that site you clicked on (in the HTTP referrer header). We call this sharing of personal information "search leakage."
For example, when you search for something private, you are sharing that private search not only with your search engine, but also with all the sites that you clicked on (for that search).
In addition, when you visit any site, your computer automatically sends information about it to that site (including your User agent and IP address). This information can often be used to identify you directly.
So when you do that private search, not only can those other sites know your search terms, but they can also know that you searched it. It is this combination of available information about you that raises privacy concerns.
DuckDuckGo prevents search leakage by default. Instead, when you click on a link on our site, we route (redirect) that request in such a way so that it does not send your search terms to other sites. The other sites will still know that you visited them, but they will not know what search you entered beforehand.
At some other search engines (including us), you can also use an encrypted version (HTTPS), which as a byproduct doesn't usually send your search terms to sites. However, it is slower to connect to these versions and if you click on a site that also uses HTTPS then your search is sent. Nevertheless, the encrypted version does protect your search from being leaked onto the computers it travels on between you and us.
At DuckDuckGo, our encrypted version goes even further and automatically changes links from a number of major Web sites to point to the encrypted versions of those sites. It is modeled after (and uses code from) the HTTPS Everywhere FireFox add-on. These sites include Wikipedia, Facebook, Twitter, and Amazon to name a few.
Another way to prevent search leakage is by using something called a POST request, which has the effect of not showing your search in your browser, and, as a consequence, does not send it to other sites. You can turn on POST requests on our settings page, but it has its own issues. POST requests usually break browser back buttons, and they make it impossible for you to easily share your search by copying and pasting it out of your Web browser's address bar.
Finally, if you want to prevent sites from knowing you visited them at all, you can use a proxy like Tor. DuckDuckGo actually operates a Tor exit enclave, which means you can get end to end anonymous and encrypted searching using Tor & DDG together.
You can enter !proxy domain into DuckDuckGo as well, and we will route you through a proxy, e.g. !proxy breadpig.com. This feature is part of our !bang syntax. Unfortunately, proxies can also be slow, and free proxies (like the one we use) are funded by arguably excessive advertising.
Because of these drawbacks in HTTPS, POST and proxies we decided to take the redirect approach to combat search leakage. However, we leave the choice up to you. You can deviate from the default on our settings page by toggling the redirect or address bar settings. You can also use our encrypted version.