The Knowledge skill in D&D 3.5 was always a little hairy. d20 Modern set out specific Knowledge skills, and Pathfinder has followed in that vein. I list them all here, with a few comments (mostly the only comments I include are along the lines of "don't use this one; it'll be worthless in my campaigns."
Knowledge (X) where X equals:
• Arcana - As in 3.5. The theory of magic, in particularly, as opposed to Spellcraft, which is about identifying practical effects. Can also be useful in identifying summoned or other extraplanar or magical creatures.
• Dungeoneering - don't take this one. You'll never use it.
• Engineering - includes also architecture, and any vaguely related field.
• Geography - self-explanatory, mostly.
• History - this one too.
• Local - as in 3.5. Great choice for a game that's set in a specific environment with no plans to travel much.
• Nature - the academic study of the natural world. As opposed to the more intuitive, pragmatic approach, which uses the Survival skill.
• Nobility - You can probably skip this one. I'll use a Local, Geography or History in a pinch.
• Planes - Don't bother. I don't plan on making this a substantially extraplanar game.
• Religion - for any check against the obvious subject matter.