In my campaigns:
Knowledge Local: You know who the local ruler is, you know local laws and common traditions and rules. And if you have been to the area before, it might help you instantly find locations that would be common knowledge. It does NOT apply to everything. It is basically a streetwise skill in my campaigns.
Knowledge Religion: You can identify depictions of deities and saints, and know them by name, and what they stand for. You know of local rituals, beliefs and religious festivals. You may also know some things about undead and angelic beings as well. The cleric in my group used this skill to identify various statues of deities, and to identify the use of a golden chariot in a church.
Knowledge Arcana: You understand magical devices, runes and rituals, and can understand how certain magical things work. You can also use this skill for information about aberrations, dragons and magical creatures. Or to identify magical factions simply by their garments. Simply put, if someone asks you if that weird magical thingabob might be dangerous, you probably know the answer.
Knowledge Nature: You know most things about plants and animals. You can use this skill to identify a plant, and figure out if that plant is poisonous. You can also use it to identify animals, know their weaknesses, and understand their behavior. And it also applies to forest creatures, such as Dryads and Fey. If a forest is dangerous, you will know with this skill. For example, a Druid in my campaign used this skill to identify that a giant centipede was agitated due to tremors caused by footsteps. And that if they all walked quietly, they could pass it undisturbed.
Knowledge Nobility: You know the names of local nobility, their customs, and how to properly behave in the company of nobles to the point where you could pass as one yourself. My players used this skill to know what exactly the rules are of a duel between two nobles.
Knowledge Geography: You know the lay of the land, and can use this skill to plan out a long journey, or determine where you are on a map. My players use this skill all the time to plan a voyage across the sea.
Knowledge Piracy: You know the names and ships of important pirates, and any rumors about them. You are also familiar with the practices and customs of pirate factions, and know anything a pirate should know regarding pirate culture. This skill comes up all the time when the players meet a new pirate faction. One of my players knew that the Stoneoar pirates did not like to be called "dwarves", which was considered a terrible insult to them. Good to know.
Bardic Knowledge: You know rare legends, folk stories, ancient myths, and legendary creatures. You may have heard tales or gossip about important people, or special items. This skill does NOT cover common knowledge, only obscure information spread through tales. The bard in my group used this skill to identify a legendary sea monster, and he knew a popular story that a famous warlord was said to have the ability to turn herself into a dragon. But he did not know if this story was really true.
