Does a site that has a robots.txt mean that if I comment on the site, my link won't pass pr to my site or count as a web backlink or is there still a benefit? I really don't know what a robots.txt do?
robots.txt controls the way how your site is accessed and who accesses it, you can stop SE or visitors etc, look for a how to in google and you'll see how many options you can have in a robots file
my dear friend....... robot.txt is the Google visiting code........... when we start seo work that time we have to invite to Google from this code.........
The "<meta robots nofollow" and "<a rel nofollow" have created a lot of confusion among some people. Robots.txt is a file that can be used to tell SE spiders and other crawlers to skip crawling / indexing certain parts of your website. Ref=nofollow for hyper links is the link that doesn't pass PR. You should read more on http://en.wikipedia.org/wiki/Robots.txt