From robotstxt.org, Source.
The rel="nofollow" is an attribute you can set on an HTML <a> link tag, invented by Google, and adopted by others. Those links won't get any credit when Google ranks websites in the search results
, thus removing the main incentive behind blog comment spammers robots.
See Preventing comment spam
on the Official Google Blog.
From that description it sounds like it only affects the ranking, and the Google robot may still follow the links and index them. If so, it is different from the robots meta tag NOFOLLOW semantics.