It is also possible to "organically" receive a link from one of those mega-spam sites, product listings, and links with no real content. The link was probably not placed by a human, and does not really benefit your site, so it will be safe to remove it.
Check out this guide: Backlink Analysis: How to Spot Quality and Toxic Backlinks
Negative SEO attack
Bad links are also part of a particularly unscrupulous strategy called negative SEO attack. At this point, everyone basic knowledge of SEO knows not to buy hundreds or thousands of backlinks, as it will undoubtedly result in a penalty on your site. Instead, some attack competitors by buying all those backlinks and pointing them to the rival's domain, making sure that it gets penalized.
The only recourse for a victim of a negative SEO attack is to disavow afghanistan mobile database all links pointing to their site.
When to disavow links?
If it's not already clear, disavowing backlinks isn't something you do on a whim. It's a pretty serious action that can significantly impact your search rankings, for better or worse.
Google considers this to be the last resort option. It's located under Webmaster Tools > Advanced and there are literally 3 warning screens you have to click through before you can upload the disavow file.
Disavowing links
In general, you should only disavow a link that you know is harmful. Check Google's Quality Guidelines for a more comprehensive list.
Keep in mind that alink from a low-traffic or low-authority domain isn't necessarily a bad link. It won't contribute much on an individual basis, but every link is a vote of confidence for your site that Google takes into account.