This is the most hassle free to send a message loud and clear. The message is that the website is not up for being associated or interested in unnecessary advertisements/links/plug-ins or any other kind of logins.
A lot of people were wondering how the search engines adjust to this tag would. The answer is simple. The search engines simply drop the target links from their web graphs, however the search engine rankings of the particular website are not affected at all.
There are basically two types usages of rel=nofollow tags.
The tag appears along with the Meta tag and can be easily incorporated with the meta tag in the HTML code. An example of rel=nofollow in meta tag is:
Another way to use this tag is on a link level. This way, the tag can be included in a href tag, as in:
This simply tells the search engine, not to follow the link to destination, thus preserving the link juice of your website. Usually this tag is added to all links in blog comments and forms.
By adding rel="nofollow" to a hyperlink, a page indicates that the destination of that hyperlink SHOULD NOT be afforded any additional weight or ranking by user agents which perform link analysis upon web pages (e.g. search engines). Typical use cases include links created by 3rd party commentators on blogs, or links the author wishes to point to, but avoid endorsing