The Deep Web, also known as the Invisible Web, is a portion of the web not reached by standard search engines such as Google and Bing. Less than 10% of the web is indexed by search engines with the remaining 90% of web content called the Deep Web. It is estimated to be 2-500x bigger than the surface web.
Content on the Deep Web is not found by most search engines because it is stored in a database which is not coded in HTML. Google and Bing might lead us to a front door [a search interface], but it generally can't search the content of a databse. It is up to you to search the database where the results of your search are loaded into a dynamically generated HTML page for viewing.
For more info, see What is the Deep Web? story from CNNtech.
Make the mental shift from find the content to find a doorway to the content.
Here are a few ways to search for content on the Deep Web: