Knowledge skills are nearly always based off of Intelligence. At a low level they can be represented by common sense and experience, but at higher levels they generally require some level of book learning.

• Alchemy - Knowledge of making, using and identifying alchemical mixtures.
• Arcana - Knowledge of magical things.
• Dungeoneering - Knowledge of things and places found underground.
• Engineering - Knowledge of how to build mechanical and architectural things.
• Geography - Knowledge of physical terrain and also where things are.
• History - Knowledge of what has happened in the past.
• Medicine - Knowledge of medicine and healing people.
• Nature - Knowledge of plants and animals.
• Nobility - Knowledge of noble etiquette, structures and who is who.
• Philosophy - Knowledge of the theories of morals and ethics.
• Planes - Knowledge of the metaphysics of the outer planes.
• Religion - Knowledge of cults, priests and gods.
• Warfare - Knowledge of military strategy.

Knowledge skills may have specialisations. Geography and History for example will have specialities for particular regions. If you have never been to, or read about, a place then you won't know anything about it until you also pick up a speciality in that area.

• Magic Theory - Knowledge of how academic magic works, for the purpose of learning and inventing spells.