The well known nofollow attribute is used in situations when you want to tell the search engine that placed the link will not be pursued. Nofollow so you can use this example that the link juice that is obtained for his side, is also retained. In addition, you can thus prevent the search engine gets some sites to see. Subsequently, they are not also included in the index.
Bloggers use the nofollow attribute and more often, so come comment spammers do not even tempted to abandon any useless comments just because of the backlinks.
Anyway, regardless of the meaning and purpose of the search engine so nofollows is informed that the concerned link is not to proceed. However, no one knows with exact certainty whether such links by the search engine will not really be followed up and not somehow crucial for the ranking are. It appears that there are search engines that do not adhere to the really nofollow.
There are many SEOs who think that nofollow links, especially for the PageRank, but also for the ranking of a page in the calculation to be included. As an example, is often used Wikipedia. For even if the total outgoing links on the page with a nofollow attribute are provided, so the links still seem to play an important role.
Summary that nofollow links are likely to be good with respect to a professional website optimization. In any case, one can say that links to ensure such quality traffic and ultimately aim to online marketing from action.