Another use for the nofollow attribute is for advertisers to use on links that have been paid for. So, if you buy an advertising banner on a website which links to you, Google says that the nofollow attribute should be added so that they know not to pass any PageRank? across that link. The idea here is that you shouldn't benefit in the organic results by buying advertisements that include links on other websites.
http://images.google.com.sv/url?sa=t&url=http%3A%2F%2Fwww.technologytimesnow.com