What constitutes "bad links"?

Bad links are considered those links which are taken from low quality content websites, or spammy websites. It is a bad practice to get plenty of low quality links such as from hacked websites etc to trick the search engine.

A bad backlink is a link that comes from a not-so trusted website. Since the Penguin algorithm change in 2012, Google has been cracking down on bad links. Before Google launched the Penguin algorithm, websites were violating Google Webmaster Guidelines with black hat SEO. Essentially, people were paying for links so they would have higher rankings and higher authority—not okay. As a result, Penguin was launched. Instead of improving their rankings, this update actually penalized webpages that were associated with these bad links.

Getting spam comment or spam or low quality backlinks(sometimes) considered as bad links.

Links erupting or generating towards a full redirection or page forwading is constituted as BAD Links. Because, it makes the Search Engine Companies impossible to decide which ranking should be allocated to the link.