They're really an enterprise search B2B play that offers site search and more contextually relevant Web search. "Contextual" here means relevant to the user population of the site (teens, moms, sports fans, etc.). They do that with clickstream analysis of aggregate user query behavior. Initially Web search is weighted according to specific business rules, but the engine learns what the group thinks is relevant to particular queries. It's a different relevance paradigm that gets "smarter" over time. But its nature, this is a vertical search engine par excellence.

They get paid on a PPC revenue sharing basis by including Google and/or Yahoo! paid results on search results pages of their publisher partners. And Eurekster has offerings for large publishers and small publishers (e.g., bloggers). The interface of the "buzz cloud" (now a "Web 2.0″ standard) is also interesting as a visual search/browse (even advertising) tool.