The term deep web is a rather imprecise term used to refer to sites and services generally not discoverable using traditional search engines. These sites have removed themselves from the search engines, either by explicit request or by including directives to prevent indexing. The intent is that only those who have a URL to such a site would know about its existence.
Since search engine removal requests and “don’t index” directives are not enforced, sites on this “deep web” can still theoretically be discovered using non-compliant search engines and spiders.
The deep web is distinct from the dark web, an equally general and imprecise term referring to sites that cannot be directly accessed on the public internet.