Robots.txt

Robots.txt file does not improve your search engine positioning.
It provides Search Engines robots information concerning which files you will not allow to be crawled and indexed in the search engines.
When the search engine robot crawls your site it looks for the robots.txt file.
If it doesn't find one it assumes automatically that it may crawl and index the entire site.

This allows all robots to crawl all files.
User-agent: *
Disallow:

This Disallows all robots to crawl a folder called /cmsbuffet/ .
User-agent: *
Disallow: /cmsbuffet/

This web site validates your Robots.txt
http://tool.motoricerca.info/robots-checker.phtml

Here is the latest explanation from Matt Cutts: