When blogs suddenly became hugely popular around 2004, there was a corresponding increase in comments being posted with the sole purpose of increasing the number of backlinks to websites, in an attempt to improve their search engine rankings (so-called "comment spam"). In response, the "nofollow" attribute was introduced as a means to indicate that a link to another website was not to be counted as an endorsement, and was therefore an important weapon against comment spam. Now, Google has introduced additional link attributes that provide webmasters with additional ways to identify the nature of particular links:
When the "nofollow" attribute was introduced, Google would discount any link marked in this way, but this is no longer the case and now all of the link attributes are treated only as hints. "We'll use these hints...as a way to better understand how to appropriately analyze and use links within our systems," said Google's Danny Sullivan. Google also explains that analyzing these link attributes may help it to understand unnatural linking patterns in the future (for example, if a website owner artificially improves their search ranking by paying for links on other websites).
According to Google, if you currently use the "nofollow" attribute on your website there is no immediate need to change anything, but since you can use more than one link attribute for any given link, you could supplement these links with a further attribute where applicable. We recommend that you ask your website designer to do the following: