Also known as the "Deep Web," the invisible web is not indexed by standard search engines such as Google or Yahoo.
Google is a great source. No doubt about that. But it has limited utility for scholarly research. Why? Google is exceptional at skimming the surface of the WWW and retrieving highly relevant results. One of the secrets behind Google's searching power is "PageRank." Simply, Google retrieves the results that have the most significant links pointing to them. But Google does not tap the "Deep Web" or "Invisible Web." Many of the high quality, scholarly sources that you need to do research are not retrieved via Google. To plumb the depths of the deep web you will need library databases, such as the databases listed in this guide.
Google Scholar (see box) is a freely-available search engine that goes beyond "plain Google" to search for scholarly literature and academic resources. REMEMBER, just because GS finds it does NOT make it scholarly, so pay attention.