The standard was proposed by Martijn Koster, when working for Nexor in February 1994 on the www-talk mailing list, the main communication channel for WWW-related activities at the time. Charles Stross claims to have provoked Koster to suggest robots.txt, after he wrote a badly-behaved web crawler that inadvertently caused a denial of service attack on Koster's server.
When a site owner wishes to give instructions to web robots they place a text file called robots.txt in the root of the web site hierarchy (e.g. https://www.example.com/robots.txt). This text file contains the instructions in a specific format (see examples below). Robots that choose to follow the instructions try to fetch this file and read the instructions before fetching any other file from the website. If this file doesn't exist, web robots assume that the web owner wishes to provide no specific instructions and crawl the entire site.
A robots.txt file on a website will function as a request that specified robots ignore specified files or directories when crawling a site. This might be, for example, out of a preference for privacy from search engine results, or the belief that the content of the selected directories might be misleading or irrelevant to the categorization of the site as a whole, or out of a desire that an application only operates on certain data. Links to pages listed in robots.txt can still appear in search results if they are linked to from a page that is crawled.
A robots.txt file covers one origin. For websites with multiple subdomains, each subdomain must have its own robots.txt file. If example.com had a robots.txt file but a.example.com did not, the rules that would apply for example.com would not apply to a.example.com. In addition, each protocol and port needs its own robots.txt file; http://example.com/robots.txt does not apply to pages under http://example.com:8080/ or https://example.com/.