A spammy link is a link on another website that is placed on or off-site which is created and published by someone other than the site owner. These links are likely to have been created for SEO gain rather than to provide genuine value to a visitor or customer.
Fake or spammy links are prevalent for a couple of reasons. First, people depend upon their Google rankings and that is the reason they create spammy links in the first place. The second reason is just that it is easy to do. Those who create spammy links never think of the negative effect it will have on their business.
Furthermore, webmasters may install spammy links to a competitor’s website or shady intermediaries may install it on the webmaster’s behalf. This occurs because the intermediary earns revenue from that page, and will usually attempt to populate it with as many hyperlinks as possible going anywhere for the purposes of SEO spam.