Google considers a natural link to be the expected end result of all the SEO and digital marketing that websites engage in, such as blogging and social media marketing. If a customer finds you through great content you have created, that is due to natural links. A link in your blog directing the reader to more information on the subject is called an editorial link, and is a perfect example of a natural link. However, unnatural links have been more loosely defined.
Generally, unnatural links are artificial links which purposely manipulate a page’s ranking. These can include purchased links, or links which spammers surreptitiously attach to your site. Lately, though, a lot of other types of links have attracted the wrong kind of attention from Google, including some which were long presumed to be innocent. This is because Google, in their Webmaster Tools document, is a little vague in setting their standards.