The robots.txt file is a small file located in the root folder of your Joomla site. The file contains instructions to the search engines on what to index and what to leave out.
Some people confuse the robots.txt file with the .htaccess file. The difference is significant. The first one only gives instructions to search engines - and most search engines respect it. The robots.txt file is not used for anything related to security. Nevertheless, it can have an impact on Search Engine Optimization.
The .htaccess file, on the other hand, is used to reconfigure the settings of your Apache server, redirect URLs and other server related tasks. More about that in a later post.