From Seo Wiki - Search Engine Optimization and Programming Languages
Yahoo! Search BOSS (Build your Own Search Service) is a Yahoo! Developer Network initiative to provide an open search web services platform. .
The main goal and idea of BOSS is to give users, in this case developers, free access to the Yahoo! Search index. The results can be supplied into the developer's website or program so that they can manipulate the resources according to their product's requirements. BOSS allows the results to be returned back in XML, JSON, HTML, text and also allows the comprehensive search feature allowed in Yahoo like pulling the results by pages, searching inside PDF, etc. The ranking of the websites for a search term is same as the Yahoo! Search ranking since both of these are pulling from the same index and ranking.
According to Yahoo press meeting, there are 30 million queries a day being run through BOSS powered web search pages.