Unnatural Links is what Google calls all links they don’t like - violating the Google Webmaster Guidelines.
Unnatural inbound links occur when somebody puts a backlink on your website to manipulate search engines and you pay them with whatever incentive.
With the help of manipulative measures like buying links a webmaster can increase its search engine rankings, acquire more visitors and increase their advertising revenues.
Unnatural inbound links can either:
Most commonly know as unnatural links are
creating links that weren’t editorially placed or vouched for by the site’s owner on a page, otherwise known as unnatural links, can be considered a violation of our guidelines.
Unnatural links are also described as being acquired in a deceptive manner. When you take this thought further, a lot of the idea of SEO is unnatural, and deceptive, as those with the bigger link budgets (or marketing budgets in general) often win the top positions.
Also advertorial links are “Unnatural Links” for many years, causing penalties like the famous Interflora link penalty.
Reminder: Google's guidelines on paid links that pass PageRank also apply to "advertorial" pages. See http://t.co/e7YluzHGas for more info.— Matt Cutts (@mattcutts) February 22, 2013
Google’s advice to avoid the links being manipulated, and thus not counted as “Unnatural Links” anymore, is to add a qualifying NoFollow attribute to the links in question.