The rel attribute refers to the 'relevance' of a link. In other words, a link tag must have some sort of relevance to the page which it is linking to. While this is not directly visible to the visitor, it is important to seach engines when spidering or crawling the site. The rel attribute influences the search engine bots or spiders by providing additional information about the relationship between the linking URLs.
The rel attribute is option and can take any of the following values:
A special rel attribute that is of particular importance to SEO is the rel='canonical' attribute. This attribute is used to identify pages on a website that essentially contain the same content. Duplicate content could be seen as unnatural by search engines and this could lead to penalties or the pages could compete against each other and dilute the URL properties of each. The purpose of the Canonical link is similar to that of the 301 redirect which is to transfer links to the canonical page or preferred URL.
Another particularly important attribute is the 'author' attribute. Google now supports the rel='author' attribute. This attribute is used to identify the author of the document (see Google for more information on authorship).