Here are my notes from Jeff Bezos's talk from the 2005 O'Reilly Emerging Tech conference, called "Vertical Search and A9." A9 has added a ton of tools to let people design their own domain-expert searches.
There are lots of things you can do if you're a domain expert in vertical search. If you're a medical pro and you search on Vioxx, you'll get different results from Web search and PubMed.
The data-sets are different and the relevance ranking and the transformations on the query are all different too.
PubMed takes the user-query and does sophisticated transformations, e.g. "Heart attack" into many medical terms.
The Web-search on Vioxx is mostly about class-action lawsuits, while the vertical is about medical info.
A9.com has a visual metaphor for vertical search — columns for web results, image results, and reference results, your bookmarks, etc…
