In simple terms, Google dominates in volume, but Yahoo! gets the most people to click on a link. Some say that this is because Google's snapshot of a web site is focused to the point that people don't need to click on the link to get the information they need. Others say that Yahoo's search algorithm is more accurate.
Whatever the case 3/4 of searches made on Yahoo! ended in a click, while only 2/3 of those made on Google resulted in a referral. This statistic also reflects a larger trend in search engine marketing -- a move towards a CPA (cost per acquisition), rather than CPC (cost per click) online marketing. Whatever the measurement (whether it's Yahoo's click rate dominance, or Google's volume dominance), marketers are more willing to accept a newer type of performance metric based on the amount of visitors who lead to a sale, or acquisition.
Yahoo vs. Google image by DannySullivan [cc, 2.0]