Deep web

The deep web is the portion of the World Wide Web that standard search engines do not index. It includes private databases, password-protected pages, and other content unavailable through ordinary search results.

Dark web: A part of the internet reachable through specialized software or configurations and associated with anonymity. Unlike the deep web broadly, it requires specialized access and is only a small portion of unindexed content.
Web indexing: The process of collecting, organizing, and storing information about web pages for retrieval. Standard engines index only the pages their systems can discover and process.
Online banking: Banking services delivered through websites or applications, typically requiring account authentication. Account balances and transaction histories are private pages inaccessible to public search crawlers.
Search engine optimization: Practices that improve a website's visibility and ranking in search engine results. Optimization can improve discoverability only for content that engines can access and index.
Surface web: The portion of the World Wide Web indexed by standard search engines. It is the searchable counterpart to the deep web.
Robots exclusion standard: A protocol through which website owners request that web crawlers avoid specified paths. Site instructions can keep otherwise public pages out of some search indexes.
Library catalog: An organized database of a library's holdings, searchable by bibliographic details. Catalog records are often returned only after a query, rather than exposed as indexed pages.
Information retrieval: The methods used to find relevant information within large collections of data. Search engines retrieve indexed material, leaving much deep-web information outside their reach.
Tor: A network and software system that routes internet traffic through relays to improve anonymity. Tor provides access to some dark-web services, not to the deep web as a whole.
HTTP authentication: Methods for verifying a user's identity before granting access to web resources. Authentication prevents crawlers without credentials from accessing protected pages.

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