Yahoo has a paid inclusion program called Yahoo Search Submit that allows advertisers put their site into the natural search results.
Search Submit Basic - designed for small sites with low budgets. Up to 5 URLs.
Search Submit Pro - for corporate level clients, with $5000 minimum to spend. It uses the classic pay-per-click model
Sites using Search Submit are usually allowed to get away with some cloaking and other shady stuff that would otherwise be punishable. Yahoo's Search Submit program generally hurts Yahoo's search results, because it forces Yahoo search technology to be more biased towards on-page criteria.
Yahoo also serves pay-per-click ads along with search results. PPC ads are on the top, bottom and on the right side. Yahoo crams up to 4 - 5 PPC ads on top, making their search engine less relevant and less information-focused.
On top of being a search engine, Yahoo serves as a portal. In fact, the earlier initial business model revolved around the portal, while search came later. Yahoo wants to be a content authority on the web, and they've spent a lot of money producing their own content. Search results are also biased towards that content. Some examples are: Yahoo Answers, Yahoo Real Estate, Yahoo Sports, Yahoo Lifestyle, Yahoo Health and a lot more.
Yahoo Acquisitions
Yahoo has made over 50 acquisitions since its birth. Some of the most notable are:
October 8, 1997: Four 11 (Rocketmail), which became Yahoo Mail), $92,000,000.
April 1, 1999: Broadcast.com, which became LaunchCase, $5,700,000,000.
May 28, 1999: GeoCities, a web hosting service which became Yahoo GeoCities, $3,600,000,000.
June 14, 2003: Overture, which became Yahoo Search Marketing, $1,630,000,000.
December 23, 2002: Inktomi, which became Yahoo Search, $235,000,000.
December 12, 2005: Del.icio.us, now delicious.com, $20,000,000.
February 12, 2008: Video on Demand, which became Yahoo Video, $160,000,000
Yahoo Optimization 
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