What is Link Spam?
Link spam is any link building technique done to manipulate search results. It is a black hat SEO technique.
In most cases, the link spammer will create many backlinks without minding if the topics on both pages are related or if the sites are in a related niche.
Link buying, automated backlinking, aggressive link exchanges, inappropriate reciprocal linking, and paid links not marked as sponsored are all considered link spam.
Google considers these link building methods unnatural and will usually hit sites involved in them with a penalty.