What is Unnatural Link

[phptraingtesting]

Recently i have read a article in which a i read about unnatural links. so i want got more knowledge about that Unnatural link.
Some of Webmaster receive the email form Google about unnatural links so share it with me.

 

[tom&jeny]

what is unnatural link?
>unnatural link is creating links that weren't editorially placed or vouched for by the site's owner on a page is known as unnatural links.

 

[aanyajack]

Unnatural Links are artificial links mainly dedicated to manipulate the page's ranking. These can include purchased links and other spammers links.