Is DuckDuckGo free?
Yes; like many other search engines on the Internet, DuckDuckGo is free to use.
How does DuckDuckGo make money?
There are a couple of different ways that DuckDuckGo supports itself financially.
One way that DuckDuckGo makes money is by displaying advertisements with some relevance to the search terms that you type in, like so:
Many other search engines use this method of generating revenue. However, DuckDuckGo doesn't like doing it (because they're all about privacy, and they feel that advertising is privacy-intrusive). So, they use some other methods of getting cash to keep advertising to a minimum.
E-commerce affiliation programs
Basically, it works like this: every time someone uses a "bang" to search for something on an e-commerce website through DuckDuckGo (see our How to Search DuckDuckGo tutorial for more on how "bangs" work), or clicks on an advertisement for one of these websites, a special code from DuckDuckGo is sent along with it. Then, if a person buys an item from that website, the code makes a note of it, and signals to the website that DuckDuckGo lead the person there. The e-commerce website then pays DuckDuckGo a percentage of the item's sale price as a commission.
In the event that this sounds a little shady to you, know that the program is still governed by DuckDuckGo's privacy-sensitive rules. The code sent with your search only records information about the item purchased, for the purpose of signalling and calculating the commission. It does NOT record any information that would personally identify you, such as your search terms, the date and time of your search, your computer's Internet address, or your browser specifications.
Other ways to support DuckDuckGo
DuckDuckGo has experimented with other monetization models, such as accepting donations through micro-donating services such as Flattr.
Generally, though, they are more interested in having people volunteer to make their service better. This could include writing "Instant Answers" to questions that people type in as search terms, or tweaking how DuckDuckGo's code works in order to provide more accurate search results.
If you're interested in what can be done to help out DuckDuckGo, click here.
That's a quick overview of how DuckDuckGo makes money!