Every link building tactic sits somewhere on a spectrum from sustainable to reckless. White hat earns trust and lasts. Black hat games the system and eventually collapses. Knowing the difference protects you from the kind of penalty that can wipe out years of progress overnight.
White hat link building follows search engine guidelines and focuses on genuinely earning links. It is slower, but the results compound and survive algorithm updates.
Black hat link building manipulates rankings against the guidelines. It can spike results briefly, then crater when Google catches up.
Most paid placements live in a grey area; not purely earned, not outright spam. They can be reasonable when handled carefully, marked correctly and kept relevant. The detail matters, which is exactly why we devote a whole chapter to it: should you buy backlinks. Whatever the tactic, judge each resulting link against the quality checklist.
If you would be uncomfortable explaining a tactic to a Google engineer, it is probably black hat. Sustainable rankings come from links you would be happy to defend in public.