Google’s link disavowal tool launched in October 2012 which allows webmaster to tell Google that they don’t want back-links from external website. Lets say an example: www.example.com is your website and you have submitted a link on www.example1.com site, now you don’t want a back link from exampl1.com, in this way Google’s link disavowal Tool will help you. Some sites want to do this because they’ve purchased links, a violation of Google’s policies, and may suffer a penalty if they can’t get the links removed. You can access disavow tool from disavow links tool page
Have a look at Matt Cutts Video talking on Removal of Low quality back-links:
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