When optimizing your web site most webmasters don’t consider using the robots.txt file.This is a very important file for your site. It let the spiders and crawlers know what they can and can not index. This is helpful in keeping them out of folders that you do not want index like the admin or stats folder.
Here is a list of variables that you can include in a robot.txt file and there meaning:
1) User-agent: In this field you can specify a specific robot to describe access policy
for or a “*” for all robots more explained in example.
2) Disallow: In the field you specify the files and folders not to include in the crawl.
3) The # is to represent comments
Here are some examples of a robots.txt file
The above would let all spiders index all content.
Here another example
The above would block all spiders from indexing the cgi-bin directory.
In the above example googlebot can index everything while all other spiders can not index admin.php , cgi-bin, admin, and stats directory. Notice that you can block single files like admin.php.