Robots.txt Tutorial
Comprehensive Robots.txt Tutorial, on how to block and allow individual robots. Etc.
The robots.txt file is used to disallow robots, (such as search engine crawlers), from visiting parts, or sections of a website.
Use the robots.txt file to disallow indexing of content that you don't want indexed, such as decorative images, borders, or button images. Etc.
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Robots.txt example
User-agent: Google User-agent: Yahoo Disallow: / User-agent: msnbot Allow: /