A "nofollow" link effectively stops a link being a link. Adding the "nofollow" attribute to a link basically informs Google that the link does not count as a vote, does not pass topical relevance or PageRank.
"nofollow" links do not pass PageRank and do not pass anchortext
Matt Cutts from Google says "Nofollow is method (introduced in 2005 and supported by multiple search engines) to annotate a link to tell search engines ďI canít or donít want to vouch for this link.Ē
Most blog commments are "nofollow" links,
Some people argue that "nofollow" links pass PR and that you cannot sculpt pagerank, I tend to agree with Google and say that "nofollow" links mark a link as non link. I think we can also sculpt PR just not precisely