The nofollow attribute is specifically designed to instruct search engines that a link should not influence the search engine ranking of the site it is linking to. The search engine will still follow the link and index the site, but it won't be used to boost PageRank or influence the ranking of the site in any way.
I'm not sure whether Google counts it as a back link (in terms of whether or not it shows the link in Google's list of sites linking to a page). Whether or not it counts as a back link, it won't help to boost PageRank or influence the ranking in any other way.
