What is the 90 rule in Spain?

Excuse me, could you please explain what the 90-day rule is in Spain? I've heard it mentioned in connection with visas and residency, but I'm not quite sure how it works. Does it apply to tourists as well, or is it mainly for those seeking long-term residency? Also, what happens if someone stays longer than 90 days without proper authorization? Could you clarify these points for me?