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.
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.