‘Invisible’ internet-based information
The web pages that can be accessed by search engines like Google, Ask Jeeves etc represent only a fraction of the pages stored on the internet. The reasons for this are complex (you can learn more from the University of California at Berkeley) but the most important from the point of view of a student or researcher is that search engines cannot click buttons or type. This sounds obvious but it is important because it means that if access to a resource requires any kind of action e.g. “click here” or “enter password” a search engine will be unable to move forward and will have to ignore the resource. If your studies require you to use databases or journals to which you subscribe it is quite likely your favourite search engine (however good) will not be able to find documents located there.