what is do follow donot follow

[john]

I have read some sites using these things for better Pr for their site can any body make me understand what are those really ?how to include in sites?

[satyakam]

The no follow tag (rel="nofollow") has been adopted by Google, Yahoo, MSN and the other major and minor search engines as a way to thwart spam. The no follow tag was originally created for bloggers who were being spammed in the comments area of their blogs with ads for pharmaceuticals, home loans and even adult ads.

These drive-by spammers contributed nothing to the blog but benefited from PageRank with links that pointed to their own websites. The nofollow tag simply says to a search engine to disregard this link as it is not to a 'trusted' source. The page at the other end of the no follow link will not benefit in PageRank. Most of the blog software and services now put in the no follow tag automatically for links in the comments area.