You know some of the secrets of arcane lore. The topics you may know about include ancient mysteries, arcane traditions, arcane symbols, contructs, the undead, dragons, giants, the fey, magical beasts, monsters, paganism, and sorcery.
Check: In addition to the standard use of Knowledge skills (as per the Knowledge skill description in the 3.5 PHB), you can identify arcane spells as they are cast or when they are already in place (as per the Spellcraft skill description in the 3.5 PHB).
Synergy: 5 or more ranks in Knowledge (Arcane Lore) gives you a +2 bonus to Translate checks involving written arcane magic. 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).