You know about social mores, customs, and proper manners. The topics you may know about include etiquette, customs, traditions, family trees, heraldry, lineages, personalities, humanoids.
Synergy: 5 or more ranks in Knowledge (Society & Culture) gives you a +2 bonus to Diplomacy checks. Also, 5 or more ranks in a Knowledge skill related to the topic being investigated gives you a +2 bonus to Research checks. These bonuses increase by +1 for every additional 10 ranks (that is, +3 at 15 ranks, +4 at 25 ranks, and so on).