No-Follow Link:

For certain links on your site, you might want to tell Google not associate your site with, or crawl the linked page from, your site.

rel ValueDescription
rel="nofollow"Use the nofollow value when other values don’t apply, and you’d rather Google not associate your site with, or crawl the linked page from, your site. (For links within your own site, use robots.txt, as described below.)

Links marked with these rel attributes will generally not be followed. Remember that the linked pages may be found through other means, such as sitemaps or links from other sites, and thus they may still be crawled. These rel attributes are used only in <a> tags (because Google can follow only links pointed to by an <a> tag), except nofollow, which is also available as robots meta tag.

Qualify your outbound links to Google