While more and more content is being digitized every day, there is still an enormous amount of information that is only accessible through what is known as the deep web, which is content that is not indexed by search engines. This includes:
The best rule of thumb with web searches is to realize that even with the enormous amount of content available, not everything is available or accessible through the web. Web searches should always be one of several search strategies - library catalogs, databases, reference resources, and archives all hold information that may be the best on the topic, but Google will never find.
The domain name of a site can give you a good idea of what content you might find there:
We know most searches for information start on Google. It's important to remember though, that Google's results are ranked by popularity, not necessarily authority. The best source for your information might not be in the first 20, or even the first 100 results. To help narrow your results from the start: