NoIndex vs. Robots.txt

NoIndex is a directive used in HTML head tags or HTTP headers to instruct search engines not to include a specific page in their index. Robots.txt is a file at the root of a website that gives instructions to web-crawling bots about which parts of a site they can crawl and index. It is mainly used to manage crawler traffic, preventing overloading of servers with requests, or to keep parts of a site private (although it is not a secure method for sensitive content).

When the NoIndex tag is applied to a web page, search engine bots can still crawl the page but are explicitly told not to index it, helping webmasters keep certain pages (like duplicate content, private pages, or pages under development) from appearing in search engine result pages (SERPs). 

Robots.txt is a file at the root of a website that gives instructions to web-crawling bots about which parts of a site they can crawl and index. It is mainly used to manage crawler traffic, preventing overloading of servers with requests, or to keep parts of a site private (although it is not a secure method for sensitive content). For public content that site owners do not want to appear in SERPs, it can direct bots not to crawl certain directories, paths, or files of a site, using the “Disallow” directive. However, because Robots.txt blocks crawling and not indexing, a URL may still be indexed if linked to from other sites.

Comparison:

While NoIndex and Robots.txt both relate to the control of content in search engine indexes, their application and results are different. NoIndex is page-specific, allowing bots to crawl but not index individual pages, and is effective when a page is not meant to be stored in search engine databases. Robots.txt, however, is a more general tool used to control the access of bots to whole sections or types of content on a site.

Using Robots.txt to block a page does not guarantee it won’t be indexed if external links to it exist, because the link’s anchor text might be used to list the URL in the index without traditional crawling. On the other hand, NoIndex ensures the content is not indexed but does not prevent crawling, potentially preserving the ability for internal pages without NoIndex to benefit from link equity passed through the non-indexed pages.

In summary, NoIndex applies at the individual page level to prevent indexing while allowing crawling, and Robots.txt is used to manage bot traffic and prevent crawling at a broader level, which can indirectly affect indexing if implemented comprehensively across a website.