The term "deep web" refers to what you cannot find with typical search engines like Google, Yahoo, or Bing. These search engines use "web crawlers" or "spider bots" to search what is called "the surface web." The surface web refers to sites that are publicly available, such as public social media accounts, news sites, shopping sites, blogs, etc. Databases like the ones the Pumerantz Library subscribes to are an example of the deep web because they are not publicly accessible.
Note: The deep web is not to be confused with the dark web. While the terms are are often used interchangeably, they're not really the same thing. The dark web is a small portion of the deep web that is only accessible through special software that allows users and website operators to remain anonymous and untraceable - which is why the dark web is often associated with illegal activity.