SEO: What is Google Adsense for search?

Google Adsense for search is a free product that lets you to place a Custom Search Engine on your site to provide users with web search. You'll also earn revenue from relevant ads on the search results page.

Google Adsense for search offers you the following:

• Choosing the content your users search: your site, a collection of sites you choose, or the entire web. You can also prioritize and restrict search to specific sections of sites.
• Customization of look and feel - you choose the colors, location of the ads, and the option to host the results on your site or have Google host them for you.
• Monetization of search results pages with targeted ads.

Reference: https://support.google.com/adsense/bin/answer.py?hl=en&answer=9879