Taken from Ask.com:

Our ExpertRank algorithm provides relevant search results by identifying the most authoritative sites on the Web. With Ask search technology, it's not just about who's biggest: it's about who's best. Our ExpertRank algorithm goes beyond mere link popularity (which ranks pages based on the sheer volume of links pointing to a particular page) to determine popularity among pages considered to be experts on the topic of your search. This is known as subject-specific popularity. Identifying topics (also known as "clusters"), the experts on those topics, and the popularity of millions of pages amongst those experts -- at the exact moment your search query is conducted -- requires many additional calculations that other search engines do not perform. The result is world-class relevance that often offers a unique editorial flavor compared to other search engines.