Hi,
To be honest I'm not really sure if the 'robots' meta tag is ever used (although Google should be obeying it), but as a result of things such as blogspam/wikispam (trying to get a higher PR) people have started using the 'nofollow' attribute on links.
As for disallowing robots to access certain parts of your site: the robots.txt file which goes in your web-root serves as a sort of access control list for spiders.
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