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As for how it differs to robots.txt well...
Robots.txt is at a site level and simply acts as a signpost saying "please don't crawl these" and list of paths, however the robots meta-tag allows you to be more granular - for instance "index,nocrawl" would tell them "index this page but don't crawl any of the links that it uses" - something which can't be done with robots.txt.
Oh and welcome to WebmasterWorld [webmasterworld.com] aroye!
...and if I do say so myself a fitting way to hit my 500th post.