Each search tool catalogues different information in different ways. A recent study from the scientific journal Nature reports that even the most sophisticated search engines list no more than 16 per cent of all Web sites on teh Internet. And the majority cover less than 10 per cent of the Net. Combined, all of the major engines cover just 42 per cent of the Web, the study reports. For evidence, simply type the same keywords into a few different search engines and survey the results.
Chances are, they'll be very different.
In short, any exhaustive search of the Net requires a sampling of a few search engines in order to get a good summary of what the Net knows about your topic. There are about a dozen major search engines, some of which I've presented below. The second important point about search engines is that they not only contain different information, but they have different rules about how they want you to search for that information. That means you'll need to familiarize yourself with the specific search criterion for the search engines you eventually decide to call your own. For a tutorial on some standard as well as little-known search techniques, visit this Search Engine Strategies page.