If you have been in the SEO business for a while you probably heard a lot about the DOs and DON'Ts about link building.
And one of this discussion topics is the Google's own rel=nofollow link tag attribute.
Google introduced this attribute to "a" tag to minimize abuse of link popularity criteria for gaining search engine ranking.
Before rel-nofollow link, search engine spiders follow any link and send the indexer if the link is found to be in compliance with a multitude of search engine criteria.
Well during those days, webmasters used to spam, blogs directories, etc to get lots of link juice so that their pages can climb up SERP.
This situation resulted in poor quality of search results.
As we know it, why Google is the giant search engine is the fact that you get what you want no matter how complicated your query is.
Google caters to users with absolutely no knowledge of search engine to experts and they may judge the results worthy or totally useless according to their need.
If Google can't meet the expectation of these users to some extent, then it's over.
So nofollow attribute introduced to discourage people who clog SERP by ranking high their irrelevant pages.
Having said that, you should know that nofollow links are not all useless.
If you have a website and get a Google webmaster tool account.
You submit your link to Google webmaster tool and start building some nofollow links and check back your dash board in the webmaster tool after some few weeks, you will see that the no follow links gets listed under links to your website from other pages.
So this may be a hint that nofollow links have some value.
Now consider one of the criteria for Google search engine ranking, link popularity.
These no follow links you have built may be considered when calculating link popularity and can help your pages gain some rank.