A robots.txt file tells search engine crawlers which URLs the crawler can access on your site. This is used mainly to avoid overloading your site with requests; it is not a mechanism for keeping a web page out of Google. To keep a web page out of Google, you should block indexing with noindex or password-protect the page.
A robots.txt file is used primarily to manage crawler traffic to your site, and usually to keep a file off Google, depending on the file type:
|robots.txt effect on different file types|
You can use a robots.txt file for web pages (HTML, PDF, or other non-media formats that Google can read), to manage crawling traffic if you think your server will be overwhelmed by requests from Google's crawler, or to avoid crawling unimportant or similar pages on your site.
If your web page is blocked with a robots.txt file, its URL can still appear in search results, but the search result will not have a description. Image files, video files, PDFs, and other non-HTML files will be excluded. If you see this search result for your page and want to fix it, remove the robots.txt entry blocking the page. If you want to hide the page completely from search, use another method.
Use a robots.txt file to manage crawl traffic, and also to prevent image, video, and audio files from appearing in Google search results. This won't prevent other pages or users from linking to your image/video/audio file.
|Resource file||You can use a robots.txt file to block resource files such as unimportant image, script, or style files, if you think that pages loaded without these resources will not be significantly affected by the loss. However, if the absence of these resources make the page harder for Google's crawler to understand the page, you should not block them, or else Google won't do a good job of analyzing pages that depend on those resources.|
Before you create or edit a robots.txt file, you should know the limits of this URL blocking method. Depending on your goals and situation, you might want to consider other mechanisms to ensure your URLs are not findable on the web.
noindexmeta tag or response header, or remove the page entirely.