The Internet is a network of billions and billions of websites and pages, and the major search engines search all of them (or at least, all of them that are accessible to the search engines). As the Web grows, so do search engine indices. That can be a pretty good thing. Good, at least until you’re searching for something very specific and are overwhelmed by a panoply of results, many or most of which are irrelevant to the original query.

Say you’re planning to go bass fishing for vacation. You navigate over to Yahoo! and search for “bass”. The first page of results refers to fishing but dedicates even more space to bass guitars, Bass shoes, and Bass Ale. But over at FishSeekers.com, a search engine dedicated to all things fishing-related, you get all bass fishing results, all the time, on that same “bass” search query.

Specialized search engines (increasingly called vertical search engines) don’t scour the entire Web looking for everything. Instead, they send out their crawlers to specific databases that contain information about a particular (and often very specific) topic or field of interest. This might include fishing, shopping, medicine, travel, jobs, real estate, veterinarians, song lyrics, and specific business channels. The list is endless...and growing. There are search engines for the websites related to human rights, cats, cartoons, UFOs, real-time flight tracking, Macintosh computers, restaurants, kids’ sites, classified ads, automotive, software, games, real estate, law, health for consumers, and health for medical professionals. There are search engines that look for content only on the mobile Web, or seek out only video and/or audio.

And that’s just for starters.

The major search engines are getting into the game, too. On Google’s homepage searches, search results can be narrowed considerably to categories such as Images, Maps, News, and Shopping. Click on the “more” tab, and you’re greeted with still more categories: blogs, video, groups, books, scholar, and finance. If that weren’t enough, there’s an “even more” tab where you can search for patents, products, search within specific topics, or even get help building a search tool for your own specific brand of community.

Vertical search is critically important to website owners, particularly those operating commercial sites, because of the mindset of the searchers who use them. When someone searches for “iPod” on a general search engine, they may be looking for how-to tips, technical specifications, or repair information. Maybe they’re just trying to grab a photo of a pink iPod for a project they’re working on. But when that same user looks for “iPod” on a shopping search engine, their intent becomes much, much clearer—and that much more interesting to online iPod merchants. By the same token, a web search for “France” could be a student working on a term paper, someone needing a map, or someone with an interest in politics or even French cuisine.

A search for “France” on a travel search engine?

Well, you get the idea. Vertical search helps reveal the searcher’s motivations and intent in a field where technology and algorithms have made enormous strides, but not to the point at which they’re reading searchers’ minds.

An equally difficult challenge to address is how do you optimize for vertical search when there are thousands (or tens, or even hundreds of thousands) of specialized search engines out there, each one different, and each applying to only a teeny-tiny subsection of what’s out there on the Web?

Beyond the all-too-obvious response: If you have high-quality, popular, vertically focused content on your site, these search engines will find you. There’s no set of one-size-fits-all answers to optimizing for vertical search. That’s the bad news.

The good news is that there are certainly guidelines to follow, particularly in several of the most important and broadest vertical categories—and these are the ones a majority of searchers are using. There might be more vertical search engines out there than you can shake a USB stick at, but overall searchers are concentrating on more popular categories, such as shopping, local, news, blogs, and health. That’s why these are the vertical categories featured on the major web search engines.

Otherwise put, there may be a pizza search engine out there somewhere. If there is, I don’t know about it. (And I’m not going to look for it, either.) Your average pizza searcher has something more specific in mind, such as she wants a pizza delivered. Now. In New York City. Preferably, in or near the zip code 10019.

Forget pizza search. This is exactly where local search comes into the picture.

Truths 30–33 explain how to determine what major vertical search categories a website belongs to and how to optimize for those major vertical search categories.