Yahoo tries to become your default search engine in various sneaky ways, such as whenever you upgrade Oracle java.
Yahoo Tries to Hook Search Users Via Java Updates - WSJ

Yahoo is also the default search engine for Firefox, for at least the next several years
Yahoo surrenders all search share gains from Firefox deal | Computerworld
although this alliance doesn't seem to have helped it gain much share.

In any case, Yahoo Overture which became Yahoo Sponsored Search is no more, but BING ADS (owned by Microsoft) is the replacement, that provides ads including Pay Per Click (PPC) on Yahoo and Bing search engines. In general the cost is lower than Google PPC, but then so is the market reach.