I found a link with following format <a href='http://www.*****.com' rel='external nofollow'> What's the meaning of "external nofollow"? Thanks
It means that the site owner instructs the SE not to follow therefor not to give value to the site the link points to.If you were to exchange links with another site and the link to your site has 'external nofollow' then the link is worthless for gaining link popularity.
rel="external" = "This is an external link" (most are) rel="nofollow" = "Do not catalogue this link" rel="external nofollow" = "This is an external link, do not catalogue it." http://codex.wordpress.org/Nofollow
External dont have significance, i think, its only for client side, to say the explorer "Hey, this link is going out of my site so please open it in new page", i think. nofollow - to tell the spiders not to follow this link, for google - it won't follow the link and won't give it pr as it should from the link source.