More computer science self-tutoring: the tutor defines deep web vs dark web in order to clarify the difference.
My understanding is that the web browsers people commonly use, find the sites they suggest by following links associated with words the client types in a query. The websites with more links leading to them appear higher in search, and likely more frequently as well.
Theoretically, a web page with no links to it can be virtually undiscoverable unless someone keys its specific address in the search bar. Such a site can be said to be in the deep web:
The collection of web resources not linked to by other sites. These resources can be virtually undiscoverable unless you know their specific web addresses.
A related topic is the dark web, which is different:
The collection of web resources not accessible by ordinary browsers because they are hosted on networks only available via particular software. These resources are purposely secret.
Deep web resources include such items as academic tables meant to be found by those who need them but of no interest to the general public.
Dark web resources include top-secret databases, bulletin boards used by citizens in countries where internet communication is stifled, and sites that enable illegal trade.
Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.